Mac OS X Hints: Use relative dates in Address Book: "Here's a little tidbit you may not have known about Apple's Address Book: when entering dates, you can use plain-language phrases and certain numeric inputs, instead of a specific date.
Macworld - http://www.macworld.com/
In Address Book, edit that person’s card, click on the Anniversary field (you may have to add Dates via the Card -> Add Field menu item), and then type next week. Address Book will populate the field with a date that’s seven days from today. If their anniversary is today, just type today, or yesterday if that was the case. If it was last week, try last week. A month from now? You got it, next month. Last month? Yep, last month. Next year? I bet you can guess that one by now…