To edit your Program dates, head over to the Programs page (this may be called something else if you've customized it, i.e., Sessions, Groups, etc.).

Click on the pencil icon on the right to edit the Program. Scroll down to the heading "Enrollment Model." Click on the pencil to edit the start/end days/dates the Program occurs.

Select this option if the Program meets on certain days every week for a period of time. Choose the days of the week, then the calendar start date and end date.

Select this if the Program meets on specific dates. Choose those dates from the calendar widget.

To add Holiday closings, see this support article.