'Ne'er cast a clout till May be out'. A clout is an old word for a piece of clothing and the saying has been in use since at least the 18th century
Some people think 'May' refers to the month but others say it means the May blossom or hawthorn which flowers in May.  In the last week most of the may flowers have come out partly due to the warm weather.   However at midday today the temperature here was only 10C due to the persistent haar (sea fog) although the warm sun appeared for a short while this afternoon with the temperature reaching 16C, but on several recent days a warm jacket was definitely needed for part of the day.  So it seems that those living near the North Sea coasts should not discard warm clothing until this month of May is out.