In America, First of May is circus slang for a newbie, a greenhorn. The circus touring season traditionally began on May 1, so someone who was only just beginning their first season would be called a First of May. The phrase is very uncommon in general parlance.
Alternatively, in the UK the phrase might be recognized as being related to Cockney rhyming slang; 'first of May' rhymes with 'gay', resulting in 'first' being a slang term for a gay person. Of course, the full phrase 'first of May' is not usually used in this sense, as that would negate the whole point of Cockney rhyming slang.
If you are looking for information on the date, you should check out May 1 or May day.