Madeira is undoubtedly a year-round destination, it has it's temperature variations, especially the higher one goes, but as a good example, I've just got back to a cold wet and windy UK after 2 weeks of undisturbed sun and daytime temps of around 27c - in late October!
To be objective, here are some tips:
Spring
If you like flowers, go and see the Flower Festival. It's associated with Easter so tends to move. Be aware that it does rain in Madeira and like the UK, April can be showery but usually a couple of days at the most.
The months of April and May are great for wild flowers and the Botanical Gardens are at their most colourful. Popular destinations can be busy, like the cobbled streets of Funchal but that can add to the atmosphere.
Summer
June is normally fine, not too hot if you have a heat sensitivity and still flowery.
The summer months, although not hot in comparison with some Mediterranean destinations can be a little sticky. But that's never a problem in Madeira; go to the unspoilt north coast or to the mountains, also a good trick if it clouds over!
Autumn and Winter
Perhaps my favourite time is during the autumn and winter months; generally good weather, perfect for walking, mild days and nights, and best of all, no cold feet. The one exception is tourist restaurants; better to avoid empty ones (especially those that specialise in fish) and go for the well-visited joints that the locals visit.
So when is the best time? All year round, depending on your preference.
For a list of Madeira's festivals and events, see here
