What do we want?
Commas for compound sentences!
When do we want them?
As you probably guessed, the example sentence is compound and thus requires a comma before the conjunction. Add a comma after wedding day.
Now, during the reception is a nonrestrictive clause, which is supposed to be separated from the rest of the sentence with commas, but since it follows immediately after a conjunction, you may omit the comma before the nonrestrictive clause. (You may add a comma before during if you want, but you don’t have to.)
It’s your wedding day, and during the reception, your partner tells you they are from the future.