San Diego's climate is famously mild year-round, and Coronado shares it. Summer (June–September) is peak season: warm, mostly dry, and busy. July and August are the best beach months, though a marine layer — locals call it "June Gloom" — often keeps mornings overcast well into July before burning off by midday.
Spring and autumn are quieter and pleasant, with temperatures in the low to mid 20s Celsius. Winter rarely gets cold enough to be unpleasant; January is the wettest month but still averages more sunny days than most of the United States.
The busiest weekend of the year is the Fourth of July, when the bay fills with boats and the beach with families. If you are coming then, book well ahead. Outside summer, Coronado has a peaceful, slightly off-season quality that suits it well — the resort town aspect fades and the residential character comes through.