Q: can I copy and past a whole day on iCal

Hi Ch6esh,

You can copy multiple items from a days worth of events to paste them for a future event. To do that, either select one or all events. Go to the drop down menu Edit > Copy, go to the day that you would like to paste the events in and click Edit > Paste. Take a look at the first link for more details.

Move events to another calendar
https://help.apple.com/calendar/mac/10.11/index.html?localePath=en.lproj#/icl100 7
 

To copy an event, select the event, then choose Edit > Copy. Select the calendar where you want the event, then choose Edit > Paste.

You can also use some keyboard shortcuts to make this work, like Command-C and Command-V for copy and paste. To make multiple selections, hold the Command key and click on each event. Those details are outlined in the articles below. 

Mac keyboard shortcuts
https://support.apple.com/en-us/HT201236​

OS X Yosemite: select files or folders
https://support.apple.com/kb/PH18939

Take care