The Karbala plains are brutally hot in summer — daytime temperatures in July and August routinely exceed 45°C, and the city's marble plazas reflect the heat without mercy. The shrine area is most pleasant between November and March, when daytime highs sit around 15–22°C and evenings are crisp.
The religious calendar matters more than the climate. Muharram (the first month of the Islamic year) and especially Ashura — the tenth day of Muharram, commemorating Husayn's death — bring vast crowds and intense mourning rituals. Arbaeen, forty days after Ashura, draws between fifteen and twenty million pilgrims, most of them walking from Najaf or further afield. Either is unforgettable but logistically demanding: accommodation is almost impossible, the city is sealed off in places, and security is heavy.
For a calmer visit, come in late winter or early spring, well outside the pilgrimage peaks. Avoid Friday afternoons if you dislike crowds — locals from across central Iraq make a weekly visit.