Wednesday!

In most places. Why? Most grocery store sales and specials start on Wednesday and many will often still have items from the previous week at the sales price so you get double the sale items. By going on the first day of sales the store is also more likely to still have all sales items in stock. Best time of day to go grocery shopping around 8pm. Stores are not as busy and you can often catch items at a discount from the bakery or deli.