The Var coast has a classic Mediterranean climate: hot, dry summers and mild, wet winters. July and August are peak season — the beaches fill up, prices rise, and the campgrounds that surround Fréjus buzz with activity. The Roman ruins and the cathedral quarter are best visited in the relative cool of morning even in summer.
May and June, and September and October, are genuinely pleasant: warm enough to swim, quiet enough to walk the streets without crowds, and the light on the red rocks of the Estérel massif is spectacular in the low sun of autumn. The Arènes de Fréjus hosts its main concert season in July and August — worth checking the programme if you're visiting in high summer.
Winter is quiet and can be rainy, but the old town is atmospheric and the Roman ruins are yours alone. Mimosa flowers on the hillsides above the coast from January onwards — one of the small pleasures of the south of France in the cold months.