Djibouti City is hot year-round, but the differences between seasons matter considerably for comfort and what you can do.
November to February is the best time to visit. Temperatures drop to a still-warm but manageable 25–30°C, the humidity eases slightly, and this is the season for whale shark snorkelling at Arta Plage — one of the most reliable whale shark aggregations in the world, running from October to February. The sea is calmer and day trips to Lake Assal and Lac Abbé are more comfortable.
March to May brings rising heat and the occasional dust storm from the interior. Temperatures reach the mid-30s. Whale sharks have departed. It's still viable for a visit but you'll be working harder against the climate.
June to September is the hottest period, with temperatures regularly above 40°C and humidity making it feel worse. The khamsin winds blow sand off the desert. This is genuinely uncomfortable and most experienced travellers avoid it. If you must come in summer, stay close to air conditioning and plan outdoor activities for the very early morning only.
Ramadan brings a different rhythm to the city. Restaurants are quieter during the day, the streets lively at night. Worth experiencing if you're prepared for the changed pace, though some facilities run reduced hours.
There are no major festivals that would particularly motivate timing a visit. The Independence Day celebrations on 27 June include military parades and public gatherings, which can be interesting to observe, but they also coincide with the worst of the summer heat.