Opening day of what in May?

I went out yesterday on a lovely little 1947 Chris Craft to celebrate the “Opening Day of Boating Season.” Given the limerick below, it’s appropriate that the boat’s name is “Flagrante Delicto.”

There’s a poem that starts out “Hooray,”
And the word that rhymes with it is “May,”
If you know what I mean,
Then I won’t be obscene,
Outdoor boating is what starts today.

My essay last year about Beltane and other early-May holidays even used “Hooray, Hooray, the first of May” as a title…but I never published the rest of the poem.