I had a customer call about year etiquette and when it was appropriate to write the year on on invitation or reply card. The rule of thumb for writing the year on an invitation is that if you have a formal event, corporate event or wedding the year should be included as a simple formality. It is also nice in the future if looking back at the invitation as a keepsake that the date is printed upon it. This is especially important for wedding invitations and formal events. As far as more casual invitations, the year is not necessary to be printed. It is not incorrect to have the year printed, however it is just not needed.
The more formal the event, the more “long hand” or involved you will write the year. For example, (casual to formal):
Saturday, May 1st
May 1, 2010
Saturday, May 1, 2010
Saturday, the first of May, Two thousand ten
Saturday, the first of May, Two thousand and ten
Remember, if you use the year in one part of your invitation, it is not necessary to repeat it in another section of your invite (like the RSVP), or on your reply card. As you have previously stated it, the year is implied and should not be repeated.