April, May, and October are the best months for Athens: warm without being brutal (20–25°C), the archaeological sites are not overwhelmed, and the city is in full daily life. June is still pleasant. July and August are punishing — often above 35°C, the Acropolis is a baking white oven, and the population of Athens itself largely decants to the islands, leaving the city to tourists and its own heat. Winter is mild and often sunny, with temperatures rarely dropping below 10°C, and the museums and monuments are quiet.
Greek Orthodox Easter is one of the great Greek celebrations and worth timing a visit around if you can. The midnight service on Holy Saturday, when the lights of Athens go dark and the churches glow with candlelight, is genuinely moving. Dates vary year to year — Greek Easter often falls a week or two after Western Easter. Book accommodation well in advance.
The Athens Epidaurus Festival runs from June to August, staging opera, theatre, and music in ancient venues including the Odeon of Herodes Atticus beneath the lit Acropolis. Tickets sell out early; check the programme at the festival website before finalising travel dates.