If you don't already know, there aren't coupon inserts in the Sunday paper every week of the year. If it's a holiday weekend, there usually won't be inserts. So how do you know when to buy a paper? It's simple. If you have access to a Coupon Insert Schedule, you'll know ahead of time how many inserts will be in which week's papers. Just be sure that before you buy the paper, you check to make sure the inserts are actually there.
Here's the 2011 schedule courtesy of Sunday Coupon Preview. Please note, I've lined through everything prior to today.