Planet's Subscriptions API is set up to make use of monthly subscriptions. 

Recurrence rules (rrules) can be used to create subscriptions that deliver during recurring periods within the total coverage time. Subscriptions API supports monthly recurrences (For example, recurrences defined using the BYMONTH property along with FREQ=YEARLY or recurrences defined using FREQ=MONTHLY). If one was to set up an rrule to deliver between March and October, delivery will occur for every possibly day in this time frame.

If an rrule is included in a subscription creation request, an end_time must be included that is within 5 years of the subscription's creation time. Start and end times should not be provided in the RRule: the subscription will always respect the timestamps included in the start_time and end_time parameters.

Example

A subscription where only imagery between March and October is delivered for images published between March 1, 2020 and November 1, 2022:

{
  "start_time": "2020-03-01T00:00:00Z",
  "end_time": "2022-11-01T00:00:00Z",
  "rrule": "FREQ=MONTHLY;BYMONTH=3,4,5,6,7,8,9,10"
}