The best period depends on how you want to spend the day. A holiday of swimming and shaded resort breaks has different priorities from a week of walks, city visits and mountain excursions.
Match the season to the activity
For an active itinerary, compare daytime conditions and the timing of inland outings. In hotter periods, keep strenuous activities away from the middle of the day and leave longer breaks. During cooler periods, consider wind and time out of the water as well as the sunshine. Mountain conditions must be checked separately from the coast.
Check the week, not just the month
A monthly description cannot predict your boat day. Check forecasts close to departure and ask the operator how changes are handled. Keep a flexible day for the excursion you care about most. Compare hotel facilities too: if pools matter to your holiday, ask the property about the specific pool and season rather than assuming every pool has the same conditions.