NOTE: To use this you must create a custom holiday event first, a tutorial for this can be found here: LINK COMING SOON!
In WoW's in-game calendar, when an important holiday event is happening the event often has an overlay so you can tell which event is happening. This is demonstrated with the Day of the Dead celebration each year. On the calendar you can see the following overlay:
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Attached to this forum post is a PNG template file that you can use to make your own calender overlays.
Once you download the template, you must first edit it in a photo manipulation tool like Photoshop, it will look something like this:
Click the image to enlarge it.
You can edit anything within the green borders of the image however you must not spill into the transparent space as it will not show up on your calendar. Once you are finished save the image using the naming convention "Calendar_EventNameHere" and convert it to a BLP file.
Assuming you have already got your DBC files ready for your custom holiday event then package the DBC files into an MPQ and place your BLP inside "InterfaceCalendarHolidaysYourImageHere.BLP"
If you followed all the steps correctly, your in-game result will look something along the lines of mine: