An open afternoon is something you can accomplish.
An open afternoon often requires decisions well before it arrives. A request was declined. Work was handed over. Something adequate was allowed to remain adequate. The blank space may be the visible result of several difficult choices.
If you finally create that space, resist treating it as a clerical error. You do not have to refill it to prove that you are useful. You can choose something spontaneous, enjoy something ordinary, or simply let the day remain lighter than usual.
When you create free time, how quickly do you volunteer it away?