OKR Periods are setup to quarters by default in Ally and they are managed automatically by Ally. For example, when a quarter ends and new quarter begins, Ally will automatically set the current period for everyone in your organization to the new quarter. All users can see the period has changed to new quarter at top of the OKR list view.
Ally administrator(s) for your organization can manage the OKR Periods. Instead of the default quarterly periods, administrators can make it monthly or any custom time period they want for their organization by adding those OKR periods, and removing the default quarterly periods.