Summer: light hydration and protection
In hot weather, lighter-textured products (for skin or hair) feel more comfortable, while sun protection and basic moisturising remain essential all the time.
Winter: deeper hydration for skin and hair
Cold weather and indoor heating pull moisture from skin and hair, so richer, more hydrating products help make up for this seasonal dryness.
Makeup changes with the weather too
A lighter, water- and sweat-resistant foundation suits summer better, while a foundation with extra hydration may suit winter better if your skin tends to get dry then.
Do not change everything at once
Gradually adjusting one or two products according to what your skin or hair actually needs is better than changing your whole routine every season for no clear reason.
Watch your skin and hair, not just the calendar
The real change in how dry or oily your skin and hair feel is the true signal to adjust, not necessarily a specific date, since everyone reacts to seasons a little differently.