You know its Monday when your pen just randomly explodes. what is the error

You're missing an apostrophe for "it's", because you are talking about a day and how it is going. When you using "its" it is talking about the possession someone/thing has. You also need to capitalize the "W" in "What" because that is the start of your question and sentence. Lastly, You need to put a question mark at the end of your question for ending punctuation.