The weather in Valletta is pleasant almost throughout the year. You will find that the summers are warm and dry while the winters are wet and cool. So depending upon the kind of traveler you are you can choose the best time according to this rough guide:
Winter: Barring the Months of December and January, that are extremely wet and humid, even winter doesn’t really pose a problem for the tourists. The temperature rarely falls below 10 degrees Celsius otherwise. If you don’t mind the cold, it is a great time to go. There won’t be huge crowds during this time.
Summer: Summer is obviously the best time to visit. It is also the most crowded. From Arpil to October the weather is sunny, the sights are bathed in beauty and the museums are in full swing with their events. At the same time, the tariffs are much higher so plan accordingly and book the flights and hotels in advance.