The AutoFill feature allows you to fill up ranges (selection of cells) with the days of the week, months,....
Auto-fill the days of the week
Enter the starting day into a cell (e.g Monday in A1)
Move the cursor at the tip of lower right corner of the cell until the latter change to a thin black + cursor (copy/fill)
Left click hold and drag the cursor along the range of cells you wish to autofill and finally release the mouse button.
Now all the cells have been consecutively filled with the days of the week.
You'll notice a small icon, namely the "Autofill options" at the end of the selection.
It allows you to perform various operations on the range of modified cells; you can for example auto fill only the weekdays.
To do so, simply click on the icon
And select the appropriate option (the modifications are automatically performed).
Auto-fill the months
The same principle applies when working with months, simply enter your starting month, click, hold and drag along the selected range of cells.
Same as above it enough for you to enter the starting date click, hold and drag along the selected range of cells.
Note that: It is important that you enter the date in the format accepted/used by Excel... (DD/MM/YYYY or MM/DD/YYYY)
The image below illustrates what may happen, if the date format is not correctly entered.