What a nice thing you are doing for your mother!

I agree, you want to avoid being too wordy, as the information you really need people to know can get lost in there. And you do have some extra info for this event.

But that also happens with clever, cute, or original wording, too. So don't go overboard there, either. After all, it's about the party, not the invitation.

I might do something like this:

***
A Day at the Races
in honor of
Muriel Throckmorton's 50th Birthday

Please join us for [dinner/lunch/light refreshments] and an afternoon/evening of ostrich races
date
time
Ben Hur Race Track
address

Please pick up your special tickets to the party suite at Entrance B
---
Dinner to follow
Chez Ritzy
address
---
Kentucky Derby attire

RSVP: copperearth@whatever.com/123-4567 [whatever you want them to use]
***

"Kentucky Derby attire" was my best shot at suggesting hats without getting too specific and bossy. If you aren't in the US, maybe "Ascot" or whatever a well-known venue where the women wear hats and fascinators would be.

Yes, I did put "50th" in. I hate when people simultaneously celebrate someone's birthday and insult them by saying something like "special birthday" instead of the year, implying that their age is something to be ashamed of. (And, yes, I am over 50!) Either "50th Birthday" or just "Birthday," just please not "Special Birthday."