First, I strongly recommend creating an external date calendar table (like a date dimension that you'd see in many data warehouse reporting applications). This will greatly increase your ability to query by different data attributes (day, week end date, month, quarter, year, epoch, etc.)

As you can add any type of date attribute to this table, it would even allow you to adjust for seaonality. (e.g. summarizing sales by Lent week 1 in 2009 versus Lent Week 2 in 2010)