Maybe the most frequent reason why after a first contact guests renounce to
come here is that they are vegetarian or have special food requirements, so
I often have to tell them "sorry, I cannot host you on that days".
(I call this article "vegetarians" since they are the most common group with
special dietary requirements, but it is valid for everyone that is not
strictly omnivore)

Why? 
Discover the reason ad see how to circumvent the problem:

Hosting people with special needs is always a challenge.
Vegetarian people put a big limit on what you can prepare for dinner.
Here there are some of the rules followed by most italian people:
- having dinner with host [and possibly the other guests] the day you
arrive, in my family that rule is deeply enforced;
- it is impolite to refuse what has been prepared, since it express an
distrust toward the cook and the host;
- other people may want local food! Here in Firenze most of recipes are 
meat (including offals, "il quinto quarto") based, there are a few veg 
ones but noone is suitable as the main course (and most of them use a
ingredient to which I am intolernat, if i eat more than two days in a row); 
anytime that I prepare a complete veg meal it cannot be a complete 
tuscan meal, and this could be unfair for someone;
- It is not nice to throw away good food.

having had many experiences I had to add these further ones:
- we share the burden to cook, so everyday cook a different person, and in
general anyone cooks on their second day of stay, since it is not good to ask
to cook someone that is tired of traveling, does not know the kitcken and
maybe could be also late, impairing everyone experience.
- over 15 thousands dinners got here, I have had 11 cases of people that got 
sick due to the food, 6 times me and 5 other guests. 
ALL the times this occurred because someone had asked the person on charge 
of cooking to "prepare a vegetarian meal because I am vegetarian" (no one 
ever got sick eating "freely chosen" meals) and they were not prepared/expert
in preparation so made errors that made food not suitable for everyone. so
the rule is "who cooks decide what to cook": you may ask and it is courtesy to
try to accommodate desires of as many guest as possible, but not at the cost
to prepare a meal that could be unhealty for someone.
- I have a mild allergy to two common veg ingredients, so I cannot eat them
too often; if I eat vegetarian (unless it is just bread and cheese) for more
than two days a week I get sick, so if have been two day with veg meal the
next day cannot be veg too. and sometime accour that i receive people that do 
not say to be veg (they are the so called "veg tolerant") that cooks veg for 
themself, but when other people cooks for them they eat everything, so has 
happend more than one time that on day 1 I had arrnged to cook veg for 
someone, then on day 2 cooked vegetarian, for me an another group arrived
that on their day 3 decided to cook veg. if next day there is a 
veg-intolerant guest, ... "gli è maiala" figuratively and materially. 
- too many times occour that people cancel very last minute, when shopping
for food has already made, this lead to have something perisheable that have
to be promply used. this could be meat based (not necessarily the meat that
usually can be frozen, but could be a vegetable used to prepare the meat dish
and only that meat dish) but also and more often just some part of the meal
that are veg and cannot be saved, so each one havs to eat that part in
excess, or having to finish it the next day, hence having already the two
weekly veg days.

Resuming in general I cannot accept vegetarian people unless the following
conditions are satisfied:
- they are "vegetarian tolerant", that is in case at last minute would not 
be possible to have a vegerarian meal, they would eat whatever has been 
prepared. This condition is enough to accept.
or
- they arrive on a day when I an 100% sure on who will cook and I can trust
that the people that is on turn to cook would cook what is arranged. This
second condition means that for certain days I can confirm acceptance only 
the evening before, whan all arrangements for cooking are made.
or
- sometime is possible to void the rule that who arrive on the same day have
not to cook, mostly when I have only one group of guests, and anyway they 
arrive at a time when not only they have the time to cook for everyone, 
but also in case they do not arrive I had the time to prepare an alternative 
meal with what I have at home or can quickly provide.  

On the other part even trusting what the people say would prepare is
not completely safe: at least two times I had people that when I asked them
for the ingredients of the meals that would have prepared next day they gave
me a list that looked as a vegan meal, then when they arrived next evening 
they pulled out of their baggage some meat and some spices that were not 
in the list and made the actual dinner as a "surprise". (I liked it, but
some non veg one would not have appreciated the spicy sausages)

In definitive I would accept veg guests only if I am 100% sure that the day
before will not be other people and so noone other than me or them will have 
cooking turn on the day the veg guest arrive, so I can arrange something for 
them, but this cannot be arranged much in advance because would be unfair 
to refuse stay to a group of 2 or 3 for two or three days, that may want to 
taste local food, and on your turn to offer their region's best one, just 
because there is one veg guest.
(and to be honest: if I have to choose between a veg and a non veg guest i
prefer the latter, since dinner will be tastier! - actually I have a couple
of veg-tol-frindly dinners, that are appreciated by everyone, but are
"almost vegetarian" [there is about 60 g total of meat for 8 people, not
separable since us used to give the taste] so for someone could fit, for
other not, tell me if it is your case.) 

This has the net result, that VF or pv tags in the calendar are a rare thing, 
Also I cannot accept non-tolerant vegetarians that need a confirm more 
than 20 hours in advance. 

About not to throw away good food.

When I prepare dinner for my guests it is very rare that there are unplanned
residuals while when I have guests and it is their turn to cook it is not 
uncommon that they did not know the right quantities, so often there are some
ingredients that remain unused;
Some of these could be more or less perisheable so I need to use it and to
find some recipe so my fridge do not get overfilled.
Sometimes shit happens and there is some people that does not show. So it
means that even if I had supplied the exact quantity, some will remain, if
the missing guest were one, no much problem, but if they where 2 or more
well, there is food for the next day, so ... could happen two things:
One that the next day guests are omnivorous, so no problem for eating next 
day, but what if next-day-guest does not eat that? 
Will it remain 3 or more days and get spoiled? 
A bad situation could also happen if were vegetarian food: if I
need to eat next day for sure the third day will not be vegetarian again (I
got sick if I eat a vegetarian-only dinner 3 days in a row, and since for 
other reasons my lunch is nearly always vegetarian, I have no other choice 
than dinner for meat). 
So even an omnivorous guest no showing could create problem to the next 
restricted one.  
Of course if next one is an omnivorous guest, I can prepare anything, so 
there are good chanches that most of stuff I have at home can be used.
If the person does not accept to eat everything, it could be a problem, 
small or big: small if I can use some of perisheable, big if I cannot use
any of the things that I have at home, and I even have to buy something 
specific, increasing the fridge filling.
Also, people that comes here comes also to taste local food, that for some
is the main reason, so when I have guests I am bound to prepare something 
local or unique. Almost all local food of my area are with various kind of 
meat, often mixed and other than bread and cheese and a couple of pasta 
dishes and desserts there are no true vegetarian local dishes that I could 
prepare, expecially as main course, respecting reasonable cost limits.

So, how to solve the problem ? Go back to the original rule: host and guest
have to share dinner. But there is no prohibition that dinner be prepared 
by the guest.
For this reason, until the fridge won't be reasonably empty I will enforce
this rule: Whoever say that will not eat anything edible that could be
prepared by other people must meet me at a time early enough [1900] to be 
able to go shopping for groceries and prepare dinner and in case they do not
show, still early enough to allow me to prepare a [free] dinner. 
This way I could accept non-omnivorous guests almost every day.

PS: A temporary, but not negligible reason why I am not very fond of
preparing a vegeterian meal is that from january 2018 went in effect a 
new law that has been called "vegetarian tax" that even if it is not a 
new tax has been applied as it was really a tax on certain vegetarian 
food, because doing that way was much more convenient to the merchants.
The law, that does not apply to prepackaged food but only to loose food,
specify that packaging material must be billed separately than the food and
cannot be given free to avoid wastage and improper disposal of them. Until  
31/12/2017 the packages were free.
Now the merchants bill as one cent each bag, that until three weeks ago gave
for free, but have not reduced proportionally the prices of the products.     
So actually they now get one cent more than before, since the cost of bags
has not changed. As a demonstartion some supermarket use to sell the same
product both loose, where customers choose and put in a bag, or prepackaged
in packages of about 700 g. Price for kg (net) is the same in both ways. 
So the cost of prepackagig (more for packege) is the same then the loose 
(where due to handling some product could be lost). If now they sell 
separately the bag, loose food should have a price about 2 cent/kg less than 
prepackaged, so the final price of 700 g loose or prepackaged were the same 
as before. no, they are getting all the gain, and when they were interviewed
for news sai that they were sorry, they could not do otherwise, but no one
told the truth, that since package is no longer a cost they could have reduced the
price of product by the same amount.
So even for this reason I do not like to buy vegetables. At least until they 
start to apply one of the tree solutions to circumvent the problem, but I do
not trust much on this, since it would be against their own interest. 
They got a gift from the sky, how could one ask them to renounce to it ?