A pop-up calendar signifies as a small calendar that appears at the side corner of the screen. The position is best defined as a part of windows that that is present at the bottom of the screen of the hardware you are working with. A pop-up calendar appears to the users when they just go for a click on the date or time present in the windows tray of the screen.
WHY A POP-UP CALENDAR IS IMPORTANT
A pop up calendar is important for the users because they are not required to open up the calendar separately whenever they are in need of it. Moreover clicking it back to resume the work they were doing. But if they have a feature of a pop up calendar they can direct click the date in between their work without facing the issue of opening the calendar separately.
WHAT ABOUT POP-UP CALENDAR IN OS X
Unfortunately in the OS X the pop calendar feature is not built-in. For OS X, the itsycal program can be used to have a calendar in the menu bar of the screen of the hardware having OS X. this program is not a new or a first program for any operating system. Moreover it is also light weight in its nature that is making them easy to work in the OS X rather than becoming problematic.
Itsycal program is executed in the OS X with the double click of the user. The developer advice the users to drag the itsycal program first in the app folder so that this program may have a permanent home in the system. 
Once the itsycal program is running into your OS X you can drag it to set its position in the menu bar. To do so the users are recommended to go for the hold command and then needed to drag the icon of the program to set it in the place where you want to set.
Once you have set the icon after dragging it you come across 2 arrows and a dot. The arrows will guide the users to check the next or previous months while the dot that is present in the center of the arrows will let the users to come back to the present month.
The furthers settings of the calendar like the date year and month arrangement and the number of weeks or months, etc. are also present in this program for the sake of the satisfaction of the users. 
So, this was all about the pop up calendar for the OS X in the form of the istycal calendar. But if the users are interested in the quitting of the program the can just go the setting panel of the icon and can quit from there. This program is of size about 1.2 MB that is actually nothing. So the users can easily have this program for the sake of a pop up calendar in their OS X.