If you haven't yet created your promotion, see Adding a Promotion.
Once you've created your promotion you can specify the days and times it will be available by clicking on the "Availability" tab.
You will see that by default a promotion is available at all times.
To specify days and times you would like your promotion to run, click "Add a new availability slot".
You will see the below screen where you can specify which days and times you want your promotion to run.
If you want your promotion to run on different times, depending on the day, you can add further availability slots to cover these days.
For example the below promotion would run from 5pm to 9pm on Monday to Wednesday, then 5pm to 10pm on Thursday and Friday. On weekends it wouldn't run at all.
Now you've set your availability, if you haven't done so already you will need to add products to your promotion.
If you're running your promotion as part of a loyalty scheme you may want to link it to the members or groups it applies to.