Calendar - OnClick to select or unselect day

I am using the Calendar control to check off days worked on a project.

User clicks on a day to say he worked on the project. It is then highlighted in green and stored in the database as a day worked on the project. However, I want to enable the user to click on the day again in case he made a mistake unhighlight the day and remove it from the database.

Unfortunately, the SelectionChanged event does not cater for twice clicking on the same day -- i.e. no event is raised.

I have hence been trying to add an event "OnClick" and it seems to be possible by adding an Attribute to each Cell in the Dayrender event:
e.Cell.Attributes.Add("OnClick", e.Day.Date.ToShortDateString());
However when calling Calendar1_OnClick I am unsure how I can actually retrieve the da ...