“Thru” is the informal spelling of “through.”
When talking about dates, “through” means “to and including.” So in a formal context, you could say, “We work Monday through Friday.” But if texting a friend, you could say, “We work Monday thru Friday.”
Don’t mix up “thru” and “through” with “threw,” the simple past tense of the verb “to throw.”
QuillBot’s Grammar Checker will fix any mistakes with threw,  through, or thru in your writing.