Wow - congratulations, you've lost 50 lbs!!!
This is a toughie, our loved ones will always want to treat us with food on our special days. You sound blessed to have a lot of friends and family that care about you.
Now that invitations have been given and plans have been made, it's much harder. My goal would have been one birthday treat meal with cake.
How to mitigate:
1. You don't have to eat pizza tomorrow. I know they're ordering it, but you've lost 50 lbs, they know you are obviously working hard. You didn't say where you'll be, but you can always bring salad or your own healthy meal. Have a sliver of cake, since it is your birthday! Not to harsh your friends, but if they are ordering a cake, it's probably a grocery store bakery cake and you know - they really aren't that good. It's okay just to have a small piece!!
2. Birthday lunch. It's YOUR birthday lunch, pick a restaurant where you can make a choice you know is healthy and delicious. P.F. Chang's (just an example) has some really yummy dishes AND they post all their nutritional info online. No dessert at the birthday lunch! Your friends will want to order it for you to celebrate your special day, but stand firm.
3. Easter dinner at your mom. Bring a gorgeous, healthy delicious spring salad (with spring greens, sliced strawberries, a few pepitas and a lite balsalmic dressing). Make sure at least HALF your plate is salad and take spoonfuls of the other stuff you want to try. You don't have to eat dessert, you really don't. Being able to say "no" to desserts in social situations is a difficult skill to master, but you can do it!! This is Easter, not your birthday!