Venice doesn't really have an off-season anymore, but it does have quieter, cheaper, and more comfortable windows if you know where to look.
The short answer
April–June and September–October are the sweet spot: mild weather, long daylight, and noticeably thinner crowds than July–August or Carnevale week.
Season by season
Spring (Mar–May) — Pleasant temperatures, blooming wisteria in back-canal gardens, and the first of the year's access-fee weekends from early April. Pack a light layer for cool evenings.
Summer (Jun–Aug) — Hot, humid, and the busiest months for day-trip crowds around San Marco and Rialto. Early mornings and late evenings are your friend; the Lido's beaches are genuinely worth it in this window.
Autumn (Sep–Nov) — Many locals' favourite stretch: warm sea, softer light, and the Regata Storica in early September. November is also when the first acqua alta (seasonal high water) events of the year typically begin.
Winter (Dec–Feb) — Quiet, atmospheric, and the cheapest time to book — but also peak acqua alta season, and Carnevale (usually mid-to-late February) briefly reverses all of that with dense crowds and inflated hotel rates.
About acqua alta
Exceptional high water is less disruptive than it used to be: the MOSE mobile flood barrier system, operational since 2020, closes off the lagoon during the highest tide forecasts and has significantly cut down on serious flooding. Minor, ankle-deep flooding around St Mark's Square is still possible on a handful of autumn and winter days — raised walkways go up automatically when it happens.
Timing around events
If you want to see Carnevale or the Biennale, book accommodation many months ahead — these periods sell out early and prices climb accordingly. If you'd rather avoid the crowds, simply pick different dates; Venice outside its festival calendar is a much calmer city.