Calendar in Excel Worksheet Locked or not working!?!?

[Egypt13]

Hello Everyone, I am not a professional excel user nor do I have loads of experience with troubleshooting it. I am a café supervisor and use a worksheet to tally up the registers at the end of the day. I have been using this worksheet for years the person whom created it is no longer here. I seriously cannot get the calendar to work anymore. I've tried right clicking it to try and see the properties. I have attached the worksheet if anyone can step by step help me with this..

Thanks!!