The best time to visit Frankfurt am Main is April to May or September to October, when temperatures are mild (12–22 °C), the city's parks and riverside promenades are at their most inviting, and major cultural events bookend the calendar.
Spring (April–May): Cherry blossoms in the Palmengarten and Grüneburgpark, comfortable walking temperatures, and the start of the outdoor café season. Crowds are moderate and hotel rates reasonable.
Summer (June–August): Warm to hot (up to 30 °C+), with long evenings ideal for the Museumsufer promenade, outdoor pools (Freibäder), and river-boat trips. The Museumsufer Festival in late August draws hundreds of thousands to the riverbanks. Peak season for prices.
Autumn (September–October): Arguably the finest season. The Frankfurt Book Fair (Buchmesse) in mid-October is the city's biggest annual event. Vineyard harvests in the nearby Rheingau make day trips especially rewarding. Crisp air, golden light, and the city at full cultural pace.
Winter (November–March): Cold (near or below freezing in January), but Frankfurt's Christmas markets — particularly the Römerberg Weihnachtsmarkt, one of Germany's oldest — run from late November to Christmas Eve and are genuinely magical. Off-peak rates apply most of winter.