Are you using SSRS or Crystal Reports? I had this issue once with Crystal. First I used data grouping to summarize records for each day of the week, but if there were no records for a particular day, that day was missing - but the client wanted to see all 7 days with zero total if the day had no records.

What I ended up doing was a separate formula for each day of the week that counts records for that day, then put all seven formulas in one report section. It was more work, but it did show all seven days and zeros for days with no records.

I'm not very familiar with SSRS, so if that's what you're using I hope this is helpful.