May 1st has been a spring holiday of some form or another for a long time. As Christianity grew in popularity it got overshadowed by Easter but some rural areas (and a handful of non-rural areas) of Europe and America have continued to have various celebrations to this day. The commies tried to take it over in the 20th century and kind of succeeded since they have the most well known May Day celebration. But not everybody celebrating May Day is a commie.
Also folks should remember that Monday is a fairly standard day for fancy restaurants to be closed. There's a host of non-commie and non-immigration related reasons for businesses to be closed today, do a little research before you start your boycott.
"May Day" as a celebration of spring is a separate "holiday". The socialists adopted May 1 in the late 19th century in honor of the Haymarket Riots in Chicago. A general strike was called on May 1, 1886, the riots and a bombing a few days later.
Probably most interesting from a legal perspective. The bomber was never identified, but 8 strike organizers were convicted, 7 sentenced to death, for the act based on the speaches they'd given to the crowd, not for any suspected connection to the bombing.