The best time to visit Cox's Bazar is from November to March, when the monsoon has ended, the sea is calm, and the air temperature is comfortable rather than oppressive. The water is clear enough for swimming at Inani and for the boat trips to Maheshkhali Island and St. Martin's Island. February and March are the driest and most reliable months.
April and May are hot and humid, with temperatures regularly exceeding 35°C and pre-monsoon storms. The monsoon runs from June through September — the beach is dramatic but the water is too rough for swimming, St. Martin's Island closes, and flooding can affect the inland roads. October is transitional: the heavy rains have usually eased by mid-month, and the beach begins to recover.
Bangladeshi public holidays — particularly Eid ul-Fitr and Eid ul-Adha — bring enormous domestic crowds to Cox's Bazar. The beach transforms completely during these periods: prices rise, accommodation fills weeks in advance, and the atmosphere shifts toward something closer to a fairground than a seaside resort. Unless you want to observe the spectacle, avoid these dates. The Bangladesh national holidays calendar is worth checking before you book.