The best time to buy a lot of groceries, especially meat and dairy, is when they are on markdown.
However, if you're like me, you may pass up a good deal because you are not really sure how long you have to get it eaten. Here's a quick guide:
Cottage cheese: good 1 week after date on container
Eggs: good 3-5 weeks after purchase
Milk: 1 week after sell by date
Yogurt: 7-10 days after sell by date
Beef: 3-5 days after purchasing, unopened
Chicken: 1-2 days after purchasing, unopened
To know specifics about groceries, check out Still Tasty's website.