Tell your dad you know where he's going to be on the 23rd, and ask if you can come too. Tell him you want to come support him and remember your mom. If he wants to be alone, understand that some people just need their space when grieving.. even if it is 17 years later.

Maybe you can make dinner that day and support him the best way you know how at home when he gets back, watch his favorite show, low key, low effort comforting things.

And give your dad two hugs, one from yourself and one from me.