Labor and labour are two spellings of the same noun. The spelling depends on the type of English.
In American English, “Labor Day” is correct.
In British English, “Labour Day” is standard.
The same difference applies to similar words, such as “behaviour or behavior,” “honor or honour,” “color or colour,” “favorite or favourite,” “favor or favour,” and “humor or humour.”
QuillBot’s grammar check tool can help ensure you’re using “Labor Day” or “Labour Day” consistently.