At our school the volunteer coordinator (a paid 10-15 hour per week position) handles it. She usually does a "volunteer tea". I believe she usually does it an hour or so before the PTO's last meeting in June. She has drinks (coffee, tea, water, soda) and snacks (cookies, etc). They also give every volunteer a notepad with some saying about volunteers on it and a pin (something like #1 Volunteer). I think she gets these from Oriental Trading - or some similar catalog. Hope this helps.
I've seen April 27-May 3 stated for this year's date (Point of Light Foundation).
At our school, annual volunteer appreciation is handled by faculty and staff, not the PTO. (They specifically prefer it this way.)
They organize a lunch where all the teachers bring the food (lasagne and salad), the Principal says a few words, and each volunteer gets a certicate that resembles a check. It says something like "Thank you - you're worth a million bucks". Anyone who can't come to the luncheon still gets their invitation and the certificate is sent later.
I really like the way they do it. As a volunteer, I appreciate a sincere "thank you", but I don't really want a huge fuss or much time and money going towards ME getting recognized.
We don't have hundreds as you mentioned, so maybe a lunch would be too much for you. Whatever you choose, I'd still suggest you keep it sincere and simple.
Does anyone know exactly when the National Volunteer Week is this year? Also, I am looking for some creative ideas for thanking/recognizing all the parents who help us with our events all year. We would be talking a few hundred people in total. Thanks!!