In the Reporter | Assistants menu | Web Assistant | Settings button | Hours Collection tab | How to collect hours?
If you set it By Month, it will default to the last day of a month. If you set it By Day, the volunteer or supervisor sets a specific day.
If your supervisors enter hours for the end of the month, for instance, but your volunteers enter them day by day, then you should set it By Day to allow that flexibility. In any case, when you pull hours reports in the Reporter the hours will be tallied for the time period you set in the report.