Custom lists are an excel feature to save time when entering data.
In the example below Monday is entered in cell B4. Using the autofill feature explained previously to copy the cells contents, it fills the other cells with the remaining days of the week and then repeats.
This works because the days of the week are stored as a custom list. To view the custom lists click Tools > Options and then click the Custom Lists tab. Notice that there are lists setup for abbreviated days of the week and months too.
It is possible to create your own custom list. Simple type the items of the list in the List entries box separating them by pressing Enter, or select a range of cells that contain the list entries and click Import. The image below shows both methods.