Federal law establishes ten "legal public holidays", including "Labor Day, the first Monday in September". 5 U.S.C. § 6103. California, on the other hand, names "Labor Day" in several statutes (e.g., Civil Code § 1689.24) but doesn't designate Labor Day as a state holiday by name. California Government Code Section 6700(a) lists 16 "holidays in this state". Although the statute identifies several holidays by name (Dr. Martin Luther King, Jr. Day, Lincoln Day, Cesar Chavez Day, Admission Day, Native American Day, Columbus Day, Veterans Day and Good Friday), it conspicuously omits the name Labor Day. Rather, it refers simply to the first Monday in September. So it seems that here in the Golden State, we will be celebrating the first Monday in September.
The significance of California designating a day as a holiday is that generally government offices are closed on those days. Droopy v. Bukovsky, 219 Cal. App. 4th 278, 286 (2013).