Hi Karen!
A piece of code – somewhere on your site – is trying to use an older piece of our own code which has been marked as ‘deprecated’ (ie, it should no longer be used).
Is it possible some custom code, your theme or another plugin on your site is interacting with The Events Calendar and is perhaps in need of an update? If you could share your system information I can take a look and see if anything jumps out.
Our standard troubleshooting steps could also be a great way to isolate the source of this notice. While right now it may not have any functional impact, in the long term it would be sensible to clean this up.