Safely alter the plist on Calendar.app
To safely alter the file just make a copy or backup of the file and then work with the original:


To modify the file log-in as a sudoer (usually an admin account) and open Terminal.
Enter the following to backup the file:

sudo cp /Applications/Calendar/Contents/Info.plist /Applications/Calendar/Contents/Info.plist.backup


you have to enter your password then.
Open nano to modify the plist content:

sudo nano /Applications/Calendar/Contents/Info.plist

now add

&lt;key&gt;LSUIElement&lt;/key&gt;
&lt;true/&gt;


after the line containing &lt;dict&gt; (usually the 5th line).
Hit ctrlo to write the changes to file. Then hit Hit ctrlx to quit nano.
Open Calendar and check if it works. (It doesn't work, Calendar crashes immediately)
to revert the changes enter:

sudo rm /Applications/Calendar/Contents/Info.plist
sudo mv /Applications/Calendar/Contents/Info.plist.backup /Applications/Calendar/Contents/Info.plist


to remove the modified file and rename the backup file.
Enter exit and quit Terminal
Change the icon of Calendar to a more pleasurable one.