The best time to visit Sur is between October and March, when temperatures are manageable — daytime highs in the mid-20s Celsius, cool evenings. From June to September the heat is serious (above 40°C is not unusual), though the coast catches a sea breeze that makes it more bearable than the interior.
If turtle-watching is your main reason for coming, the nesting season runs from May to October, with peak numbers in July and August. This coincides with the hottest weather, so plan accordingly — visit the beach at night when it's cooler and the turtles come ashore.
Oman observes the Islamic calendar for public holidays and Ramadan. During Ramadan, daytime eating and drinking in public is restricted; restaurants open after sunset.