In case if you have some Marketing campaign and you want that Membership for that campaign will end on some certain date, then you need to link it with period.
How to do that:
1. Go to System->Periods and create a new period:
2. Create a Period line and give it start and stop dates. Make you stop date at the end of the campaign. You can also exclude some days of the week, when this campaign shouldn't be applied:
3. Go to Products->Memberships. select the membership that you want to change and select the period you just created:
By default, all memberships are linked to period 'All', that means it is valid all the time, and only expiry date defines when it will be expired. When you linked it with some certain period, this means that membership will be valid only when this period is valid, no matter for how long time you sell it.