The best time to visit Dubai is from November to April, when the weather is pleasant and you can enjoy the outdoors. Occasional rainfall occurs in December and January, which can keep temperatures in the low-20s. In January and February, the city comes alive with shopping festivals, where you can find fabulous discounts. The warm, sunny days of late-fall and early-spring are just about as perfect as weather can get.
June through September, on the other hand, are unbearably hot and humid, and Dubai is best avoided during this time (though the crowds will be thin). Temperatures regularly approach 45 degrees. Also keep in mind that during Ramadan, shops and businesses may alter their timings and most restaurants are closed during the day.