I'd make a "Date Dimension" table, with one record for each day of the year. Then, you have a field for each additional attribute you want to track. In your case, they would be Week_No and Month, but you could also add other attributes later if you wish, like Day_Of_Week, Weekend (Yes/No), Holiday (Yes/No), Fiscal_Quarter, etc. etc.
Link your entry table to the Date table using the date field, and you can easily add up the entries by week, month, quarter, day of week, or anything else you'd like to track.