Spring (April–May) and autumn (September–October) are the best times to visit Istanbul. Temperatures are comfortable, crowds are manageable, and the city feels at its most livable. April is especially good — cool enough to walk all day, warm enough to sit outside in the evenings.
Summer (June–August) is hot, humid, and crowded. Temperatures regularly hit 30°C and the major sites can be overwhelming. That said, if you're visiting the Bosphorus or the Princes' Islands, summer has obvious appeal — the ferry rides are glorious and the waterfront restaurants come alive.
Winter (December–February) is cool and wet, with January averaging around 8°C. It rarely snows, but when it does the city looks extraordinary. Museums are quiet, hotel rates are low, and the city belongs to its residents again.
Ramadan shifts annually; during the holy month, some restaurants in conservative neighbourhoods keep reduced hours before iftar (sunset meal), though Beyoğlu is largely unaffected.