When Typos go Bad: AAA Brief Grammar Lesson
This is one of my grammatical pet peeves, and I'm shocked that it actually made its (not it's) way onto a piece of marketing material! The reverse side also has it, so whoever (not whomever) proofread this thing must have thought it was correct.
Your -- a possessive pronoun for the second person, i.e. your postcard, your invitation, your mistake.
You're -- abbreviated version of You are, i.e. You're Invited, You're Mistaken, You're Embarrassed.