Azerbaijan's climate varies more than its compact size suggests, ranging from the mild, humid coastline around Baku to the drier steppe further inland and the cooler mountain valleys near Gabala and Sheki, so the right time to visit depends partly on where in the country you plan to spend most of your trip.
Spring, from April to May, is one of the most comfortable windows for exploring Baku on foot, with mild temperatures for walking the old city and the Flame Towers promenade before the summer heat and humidity set in. It is also a good period for visiting Gobustan's rock carvings and mud volcanoes, since the site offers little shade later in the year.
Summer, June to August, brings hot and humid weather to Baku, with temperatures regularly climbing into the mid thirties Celsius; travelers visiting in this window often shift sightseeing to the morning and evening and use the Caspian coastline or a side trip to cooler Gabala to break up the heat.
Autumn, September to October, mirrors spring in comfort and is widely considered the best overall season, combining pleasant daytime temperatures with fewer crowds at major sites and good conditions for the drive up to Sheki and its former Silk Road caravanserai.
Winter, from December to February, is mild by regional standards in Baku itself, rarely dropping below freezing, which makes the capital a viable year round city break even if some outdoor activities further inland become less appealing. Snow is more likely in the mountains around Gabala, occasionally allowing for light winter sports in a good year.
Whichever season you choose, book accommodation in Baku's old city early around major events such as Novruz in March, the country's most important holiday, when hotel rates rise and central streets fill with public celebrations.