We are talking from polar circle to jungle. My colleagues in Finland have different weather if they live 500 km to north.
In southern Finland snow melts away at the beginning of August. All snow is gone usually before first of May. Willows starts to bloom usually first of May. It also means that ground frost has melted.
It is end of May when apple trees, dandelions, mapples and many other garden plants starts to bloom. Also swift
comes to Finland, and it is summer then (Apus apus). I have 15 birdhouses for them.http://www.avesphoto.com/website/pictures/SWFCOM-1.jpg
Bees starts brood raising when they get pollen from willows at the beginning of May. It takes to 2-box hive 1,5 month that it is big enough to gather honey. If hive is 5 frame after winter, it takes 2 months to get itself to get big enough.
Usually we can add first super (third box) at beginning of June.
During 12 years I have feeded pollen to hives to start them to raise brood 3 weeks before normal. When willow start, big numbers of new bees are hatching. Hives get a good false start compared with natural beekeeping.
May is quite cool. On some week bees can fly and on some it is so cold that they cannot. If it does not rain in May it will be lack of water in summer.
Farmers starts their field works at the beginning of May. It depends how it rains or how fields became dry. In August fields are in frost.