@Liz51
This is only possible by special offer (which a would prefer anyway, simpy mention in your listing extra discount can be offered if booking for more then a month in a certain season)
or
(if having the multicalender availble) if you create a ruleset with "minimum nights to stay" set to a high level (for example 90 nights) in combination with extra discount set (or a custom price set), and then apply the ruleset to the specific period (season)
Without multicalender:
You can set custom prices for specific months, but then still also need to set minimum stay to high level.for specific period (=additonal requirements in menu "availibilty" under "trip length")
But you need to be aware both options will not allow guests to book for a "short term stay" in that specific period.
Best regards,
Emiel