Use it every single day, rain or shine. For extra protection against free-radical damage, look for a daily moisturizer that also has antioxidants (like vitamins C and E).
Great Bets: Aveeno Positively Ageless + Antioxidants SPF 42 ($20; drugstores); Clinique Superdefense SPF 25 Age Defense Moisturizer Very Dry to Dry ($44; Clinique.com).
If you're letting a few veins, blotches, and bumps stand in the way of summer's fluttery little skirts, cool shorts, and the deliciousness of warm breezes on bare skin...stop right now and read this.