Invite wording- QUICK HELP!

We're ordering invites now (online)...

Should it say...


at half past six in the evening

or

at six-thirty in the evening?

Thanks!!!

also, if 'a half past six in the evening' is correct, shold the 'a' be in there?

How formal is the rest of the wording? I would say that your choice in the other text dictates this choice.

thanks everyone!