Nicosia is an inland city without the sea breeze that moderates temperatures along the Cypriot coast. Summers are genuinely hot — July and August regularly reach 38–40°C, making sightseeing on foot in the old city uncomfortable during the middle of the day. If you visit in summer, do your walking in the morning and late afternoon and retreat indoors for the midday hours.
The best times to visit are spring (March–May) and autumn (September–October). Temperatures in these months are warm but manageable — typically 20–28°C — the light is excellent, and the city is not overrun with tourists as the coastal resorts are in summer. Winter (December–February) is mild by Northern European standards (10–15°C, occasional rain) and perfectly manageable, with the added advantage that the museums and old city are quiet.
Easter (Orthodox calendar) is a significant event — the streets of the old city fill for the Good Friday procession, and the celebrations are worth timing a visit around if you can.