You obviously know that you copy / paste a cell by clicking on its lower right corner, and dragging it down or across while holding the left mouse button down.
For some cell values, e.g. dates or numbers, you can even switch between an incremented copy and a straight copy by holding or not the Ctrl key. By default, dates will increment and numbers won’t, and holding Ctrl will switch the behavior.
NOW, if you perform this dragging with the right-click instead of the left-click, you’re getting a lot more options (see screenshot below), like filling in weekdays, or months. This is quite the time saver.
Try it, you might like it.
PS: Try selecting multiple cells before dragging and see how that affects the results you’re getting.