In order to do that you have to link your page with period. By default if it's not linked with any periods, it will be always shown:
If you only want to show it only for certain days between some time (for example on Monday and wednesday between 23:00 and midnight), then you need to follow the steps:
1. Add new Period "happy hour". Go to System -> Periods. Add new record here.
2. Add new period line. You can add multiple lines for the same period. This way you can show the page every monday between 12:00 and 14:00 and every wednesday between 23:00 and midnight.
3. Use "Next day" checkbox to specify that period should be ended on next day.
4. When you're done with period configuration, open Pages in Product->Pages, select the page, open "Periods" tab and check the period that you just added.