The short answer is any time is a good time to visit Malta and Gozo.

In general spring is a great time to visit Malta for the following reasons:

The best weather of the year – the temperature would be from 20 degrees celsius, with mostly sunny days. In May some people start swimming but April onwards is good for actually visiting the beaches.
The most popular for annual events – we mention plenty of annual events is which you can see in the different months, but events such as the International Fireworks Festival, Festa Frawli, Isle of MTV, Medieval Mdina, Earth Garden and even the local feasts start to ramp up.
Beat the heat and peak tourist season – as in most countries in Europe, July and August are really peak becoming incredibly busy. This also happens in Malta, so coming earlier beats most of that. July and August gets pretty hot, so for people not used to the heat, this gets a bit overwhelming.
Malta at it’s most beautiful – given that the country is pretty dry in summer, you’ll find that most greenery is right after the country is coming out of the windy winter months, the countryside is in full bloom in April / May.
But with a fantastic climate, over 300 (three hundred) days of sunhine and more than three thousand hours of sunshine a year couple with mild winters, any month is a good time to visit the islands.