Union Day?

I see that Robin Vos and Van Wanggaard are both considering ditching the Labor Day festivities. That is apparently because the unions don't like either of them.

Republican Pam Galloway is unwelcome by the unions in the Labor Day parade up in Wausau.

Since when did Labor Day become Union Day? I though the holiday was for the purpose of honoring the working people of the nation upon whose backs all this oppressive government spending is dependent. What has that got to do with unions?