What about preparing tortillas with many kinds of fillings? You could have some beef, shredded chicken, cheddar, corn, beans, tomatoes, salad, peppers, guacamole, salsa, sour cream, chopped cilantro etc. - you name it. You would have to do quite some preparing in advance, but most of it can either be served cold/at room temperature or be reheated in the microwave. Plus it's fun to assemble at the table, and it's communicative ("Could you give me the corn?"). We had recently had it with 10 persons and it was awesome.
But it's not really formal.

i haven´t made this lasagna but it sounds fantastic (and worthy of the occasion): http://www.nytimes.com/2011/11/27/magazine/gooey-wild-mushroom-lasagna.html

You could make something that improves with a 24-hour sit - I'm thinking perhaps a Bolognese? You could make a huge batch, and then just rewarm it a few hours before the party - bolognese can just sit and sit, it's only going to get better. And pasta is easy enough to throw into water when everyone's ready. Throw together a few bags of salad on the side, a few loafs of bread, and voila! Not a lot of work, especially the day of the party. It's my go-to big-party meal.

Of course, chili is another meal that serves a lot, can simmer unattended, and improves in flavor over 24 hours.

Brisket keeps amazingly well and would actually be better a day or 2 after you've cooked it so it could definitely be prepared ahead of time.

Happy Birthday!

i like the idea of a big batch of pulled pork and coleslaw- both are better after they sit. then you can just pick up a bunch of rolls and some sides and you're done!

Quiche are easy, keep well, and can be served cold, room temp, or hot. You could make two or three different types for different tastes.

I second the pulled pork suggestions--I *just* now stuck a pork shoulder into the slow cooker to make BBQ pulled pork for our Valentine's dinner. We'll eat it again later in the week--when I suspect it will taste even better!--and freeze the leftovers.

Also second the bolognese idea. Last night I pulled out some bolognese from the freezer that I'd made a week or two ago from a slow-cooker recipe on this site, and it was great--actually much better than it was the first time around!

Hello - Abigail here! Thanks for all the suggestions - all of them are more inspired than the chicken I had in mind. If it helps, the party will be very informal - buffet-style presentation and standing or perching to eat. We're not posh, us.

Second the brisket! I just made an overnight brisket for our daughter's birthday party, which was a lunchtime event. Rub the morning before, dropped in the slow cooker the night before, and it was delicious. http://www.epicurious.com/recipes/food/views/Braised-Brisket-with-Bourbon-Peach-Glaze-388709 -- I followed the braise, but shredded the meat for sandwiches instead of doing the glaze.

The chipolte beef recipe (posted two weeks ago) can be made ahead and was very tasty the next day. I was thinking of making it for a taco party with spanish rice, beans etc. - good for a buffet. I don't have a crock pot, but used the oven at a low temp for a few hours.

I agree with the pulled pork/brisket. I've done a lunch for 80+ with pulled pork, coleslaw, rolls and salad. Everything but dressing the salad is done ahead.

I would also recommend lasagna and soups/chilis. Both can be made ahead and reheated and served with a simple green salad and bread.

If you are willing to put a little more time in, make your own tacos are good too. Prep the fillings ahead of time and store in the fridge. Just before serving reheat the warm ingredients (shredded chicken, etc.) to serve.

If you want something a little classier, perhaps a poached salmon would work?

If you have access to a big crockpot or two, chili is always a winner. Or you could do it on the stove the day before and just reheat. It can just sit and simmer until you're ready for it. Serve it with salad, cornbread, and toppings (sour cream, cheese, green onions, corn chips, etc.).

I'll add another vote for the brisket. It isn't too labor intensive, cooks well, is easy to eat if you are balancing a plate on your lap and is special enough for a special occasion. While the tacos are a favorite in my house preparing them for a crowd with all sorts of variations, and accompaniments means prep work (not elaborate but nevertheless prep work) and washing of all the bowls / utensil.
Good luck and Happy Birthday (and don't worry that number isn't so scary!)

Pulled pork in the crockpot! Serve with buns and coleslaw. You can make the pulled pork in advance, then shred it and put it back in the crockpot on warm.

Cheap and easy.

Meatballs in gravy po boys. You can make the meatballs ahead, store in the fridge and reheat. The ones I made, still tasted delicious like the day they were made, after being in the fridge for a week. Americas Test Kitchen just did a show on the perfect meatballs, you can find the recipe on their website. I made a simple brown onion gravy that the meatballs simmered in for an hour.

Our huge extended family enjoys doing a "christmas curry" celebration at the family reunion. Curries don't have to be spicy and there is a huge variety of flavors and ingredients in pan-Asian cuisine. If you have a rice cooker, making and keeping rice warm is a breeze and the curries can all be made before hand. Every time we've done a curry potluck party people have had a blast!

Braciole is a fabulous kind of hearty dish that impresses folks and works great with some pasta. Braciole, how I have experienced it, is a stuffed flank that bakes slowly in tomato sauce for several hours.

We also have a large family and for one big get together, I made 2 lasagnas - one vegetarian (Cooks Illustrated's mushroom) and one with meat. Both can be made ahead of time and then slowly reheated in an oven.

Abigail here again. Thanks for all the brilliant suggestions - I''m now spoiled for choice instead of drawing a blank! On reflection, I think I'm going to go for the pulled pork or the brisket, depending on what the butcher has, and which is cheaper.