You can't set different cancellation policies for different dates on the calendar.
If you change your policy to Strict and take reservations for Coachella and Stagecoach 2019, then those specific reservations will have Strict cancellation, even if you later set your policy back to moderate. But then *all* reservations taken while your policy is Strict will have Strict cancellations.
(The cancellation policy for any given reservation is whatever was in force at the moment the reservation was made, regardless of the current setting.)
So the nearest thing to do is set your Cancellation policy to Strict, open the dates for Coachella and Stagecoach 2019, and hope they book up first and quickly. Then set your policy back to Moderate for everything else.