Tandil sits in the Buenos Aires province interior, where the climate is temperate but with sharp seasonal swings. Spring (September–November) and autumn (March–May) are the most comfortable times to visit — temperatures in the 15–25°C range, the hills green from winter rains, and fewer crowds than the summer peak.
Holy Week (Easter) is the city's most celebrated time, when the hillside Passion Play draws tens of thousands of pilgrims and spectators from across Argentina. Accommodation books out weeks in advance and the city has a charged, festive atmosphere quite unlike the rest of the year. If you want to attend the play, plan well ahead; if you'd rather have the hiking trails to yourself, avoid this week.
Summer (December–February) brings reliable warmth and long days, but also the Buenos Aires escape crowd. Tandil is a popular long-weekend destination from the capital, and prices and visitor numbers rise sharply from late December through Carnival. Midweek visits in summer are noticeably quieter. Winter (June–August) is cool and quiet — occasional frosts, shorter days, but the countryside has a spare, satisfying quality and the parillas are at their best.