Najaf's climate is desert, with very hot summers and mild winters. Daytime temperatures from June through September regularly exceed 45°C, and the sun is brutal. The best months for a visit are November through March, when the days are cool and the evenings sometimes cold.
The major Shia pilgrimage seasons — Muharram (the first ten days, especially Ashura), Arba'een (forty days after Ashura), and the Mawlid of Imam Ali — bring vast crowds to the city. The streets fill with millions of pilgrims walking from Najaf to Karbala for Arba'een, which is one of the largest annual human gatherings on earth. It is an extraordinary thing to witness, but accommodation is impossible to find and movement through the old city slows to a shuffle. If you want to see the shrines without the crush, come a few weeks before or after.