We all know that April showers bring May flowers, but Greeks celebrate May 1 (also known as May Day, Labor Day, and Protomayia) with the enchanting Feast of the Flowers.
Revelers flee to the countryside on this national labor holiday to herald spring. By May 1, most of the Greek islands are warm with gentle breezes and the mainland can even get hot. It’s a marvelous day of picnicking and flying kites and enjoying nature.
People spend the day collecting flowers and turning them into wreathes. There are even several flower festivals throughout Greece.
Stephanie Nikolopoulos 
Since 1st of May is tomorrow, I’d like to wish you Kali Protomayia! And a small correction if you permit: We don’t fly kites on Protomayia, as far as I know, at least in the mainland of Greece.
Kali Protomayia!
According to Rough Guides, children fly kites:
http://www.roughguides.com/travel/europe/greece/public-holidays.aspx
Perhaps it is regional, though, because you are right that it’s not as common. I think it’s more common for Clean Monday.