Spring (April–June) and autumn (September–October) are the best times to visit Çorum and the surrounding Hittite sites. The temperatures are comfortable for walking around open-air ruins, the light is good for photography, and the sites are quiet — Hattusa in particular gets crowded in midsummer but is often nearly empty in May or October.
Summer (July–August) is hot and dry, with temperatures regularly reaching 30°C or above. The archaeological sites are exposed and shadeless, and walking the full circuit at Hattusa in July afternoon heat is punishing. If you come in summer, start early — be at the gates by 8am before the worst of the heat. The Museum is a good afternoon refuge.
Winter is cold and the sites can be muddy, but the ruins are completely deserted and there's something to be said for having Hattusa entirely to yourself in grey winter light. Snow is possible from December through February. Most facilities in Boğazkale village stay open year-round.
The leblebi chickpeas that Çorum is famous for are available year-round — no seasonal considerations there.