The best time to visit Abu Dhabi is from November to February when it is cool to cold and one can really make the best of the outdoors with leisurely strolls within and around the island. The temperature during this period usually ranges from 9 to 25 degree centigrade. Warm clothes such as a jacket or shawl are useful as it is quite breezy. Overcoats, semi-formal jackets, boots and pullovers are common sights during this period. The malls are full of autumn-winter collections.
Winter sales start around 26th of December but not all stores. However this is a good time to visit. The Dubai Shopping Festival (DSF) is from 15th January to 15th February and most shops in Abu Dhabi offer discounts during the same period (and sometimes beyond). Some people recommend that you visit the stores in the first week of SALE so as to get the bargain on the best stuff and to get the right sizes. Towards the end of the sale, the remaining goods are reduced further (50% to 70% off is usual) and it is possible to get some great bargains.
October, March and April are transition months which are best for beaches when it is not too hot to head out . These months from November to April also sees Abu Dhabi in its best form. Roads and parks are laden with colorful flowers and it is a beautiful sight…considering that it is after all a city in the desert!!
Peak summer starts from late May with July-Aug being the hottest months with temperatures in the range of 100-120 degrees Fahrenheit (up to about 50C). Humidity is very high through to September and it can be the most unpleasant time to be outside. The evenings are slightly cooler but the breeze is still very hot (warm is an understatement) with just a slight dip from the day temperature. The heat can surprise you when you step outside from an air conditioned hotel or mall. Unless you are keen on sunbathing and shopping in air-conditioned malls, (there are usually sales in July) the best time to visit Abu Dhabi would be during the cooler months. Besides a Dune Dinner and Desert Safari tour (which is usually in the itinerary of most visitors) is much more pleasant when it is not too hot.
Abu Dhabi is a modern place where trendy fashion labels are commonplace. Loose lightweight clothing is ideal, particularly during summer. Tight or revealing tops and short dresses are inadvisable, especially on roads, in taxis and if walking about at night. At big hotels you can be dressed as you please but do keep in mind that traditionally the local people are covered and many of the non-local residents are also strict Muslims. So short, revealing or tight clothing is considered disrespectful. It is best to cover your shoulders and wear skirts/pants/trousers at least to the knees.
On the beach, a normal bathing suit or bikini is fine, but avoid 'thong' bikini bottoms or topless sunbathing. Even if you do not personally believe that it's a big deal, you are a guest in someone else's country, and you must respect their beliefs.