Drupal 7 Calendar - How do you modify link from day number? When I click on the day number, it currently goes through to day page.  I'd like to customise this to go to a specific event page for that day.

I can't find a hook for this.  Is there one?
The day page is generated by the day display of the view generated by the calendar module. You can customize this display as much as as you want - add header, footer, embed an attachment view, or otherwise enhance it:



You can disable the day display if you don't want the links at all.

Replacing it with another page would be much harder, but it might be possible using the panels module.