Grocery stores put sales on items in certain seasons. Stock up when they are on sale and you can save a bundle.
Frozen Food - March is frozen food month. Always great sales.
Dairy - June is national dairy month, which is not quite as big a deal as frozen food month.
BBQ Products (ketchup, mustard, relish, salad dressings, napkins, etc.): Early in the summer. These companies want to fill your pantry so you don't buy competitors' products all summer. Good time to stock up a year's worth.
Beverages (soda, beer, juice drinks, apple juice): All summer, especially around holidays such as Memorial Day, Labor Day and July 4th. Also usually good prices on large items (like two-liter soda) during holiday sales in other months.
Baking needs (flour, sugar, corn/vegetable oil, aluminum foil, etc): Thanksgiving and December holidays. This is also a good time to stock up on holiday-related items you like to eat all year, such as cranberries and canned yams.
Italian products (ricotta, mozzarella, crushed tomatoes, olive oil): Christmas, Easter and Columbus Day. Holiday stockup goes for all ethnic products, such as Chinese products at Chinese New Year, Jewish products during Rosh Hashanah and Passover, etc.