Malta's climate is reliably warm and sunny, with over 300 days of sunshine a year. The flip side is that the summer months can be very hot — July and August average 32°C — and the beaches and historic sites get genuinely crowded.
Spring (April–May) is arguably the best time to visit. Temperatures are pleasant (18–24°C), the countryside is green before it turns brown, and the crowds are manageable. Wildflowers are out across the limestone terraces.
Summer (June–August) is peak season. The sea is warm and festivals are in full swing, including the village festa season. Accommodation prices are highest and popular sites book up fast. The Hypogeum in particular needs to be reserved weeks ahead.
Autumn (September–October) is another good window. The heat breaks but the sea stays warm well into October, crowds thin out after mid-September, and the island settles into a more relaxed pace. Notte Bianca in October is a highlight.
Winter (November–March) is cool and quiet. Rain falls between November and February, sometimes heavily. The sea is too cold for most people to swim. That said, Valletta and Mdina are uncrowded, accommodation is cheap, and it is perfectly pleasant for walking and sightseeing. Winter is also when you are most likely to have the prehistoric temples to yourself.
Malta has no real shoulder season as such — even in winter, tourism continues year-round. Book accommodation in advance whenever you go.