São Miguel is mild year-round — the Gulf Stream keeps winters warmer than you'd expect at this latitude and summers cooler than mainland Portugal. Rain is possible in any month; the Azores sit in the middle of the Atlantic and the weather is famously changeable. The local saying is that you can experience four seasons in a single day, and it is not always an exaggeration.
May to October is the most comfortable period. July and August are the driest and warmest months, with temperatures reaching 24–26°C, though the island fills with Portuguese tourists in August and accommodation prices spike. June and September offer a good balance — still mostly warm and clear, fewer crowds, and lower prices.
April to June is when the island's famous hydrangeas bloom along the roadsides, turning the lanes an intense blue and pink. If you are visiting for the landscape, this is one of the most photogenic times to be here.
November to March is quieter and wetter. Whale watching is possible year-round but peaks in spring and summer when sperm whales are most reliably present. The Festa do Senhor Santo Cristo dos Milagres — the most important festival in the Azores — takes place in May, drawing huge crowds from the Azorean diaspora worldwide.
For the best mix of weather, fewer crowds, and reasonable prices, May–June and September are the sweet spots.