Gramado is at its busiest from late October to mid-January (Natal Luz) and again in mid-August (Festival de Cinema). If you want the lights and crowds, those are the dates. If you want a calmer visit, March, April, May, and September are excellent — cooler than the rest of Brazil, with green countryside and far fewer tour buses.
Winter (June to August) is the other major draw. The temperatures dip close to freezing at night, and very occasional snowfalls happen — a serious novelty for Brazilians. Pack warm clothes; central heating is rare. Summer (December–February) is mild compared with most of Brazil, around 25 °C in the day, and a popular escape from the coastal heat.
Hortênsia season — the hydrangeas that line the streets and lakes — peaks in November and December.