Daegu's valley location gives it the most extreme climate of any major Korean city. Summers are hot and humid — famously so — with temperatures regularly above 35°C in July and August, earning the city nicknames like "Dafrican" (Daegu + Africa) among Koreans. Spring (March to May) and autumn (September to November) are the best times to visit, with mild temperatures and good visibility.
Autumn on Mount Palgong is one of the better foliage displays in the Gyeongbuk region, and the hiking is at its best from late September through October. Spring brings cherry blossoms to the city's parks and roadsides, with the concentration at Suseong Lake particularly good.
Winter is cold but dry, with occasional snow. The city functions normally year-round; there is no particular reason to avoid winter unless you dislike the cold.