Seasonal variation in custom fields

You can't adjust the template itself but you can send a different template using a trigger that only targets certain months of the year. To do that, create a trigger and select where the Arrival Date is > start date and also Arrival Date < end date. That will only send to bookings that have an Arrival inside that date range. Or target the season if you have seasons, however seasons might be "specific date" seasons that end this year and don't work in the future.