Not too long ago, the hours of the Clark Lake Post Office changed. Here’s the latest. Window hours are Monday through Friday, 10 am – 1 pm and 2 pm – 5 pm. Saturday 9 am – 11 am. If you are a boxholder, your mail should be ready by 11:30 am, Monday through Friday. The lobby is open 24/7 to access post office boxes or to mail letters.