To get the day of the week in a cell you have two choices. You can use a format or a formula.
You can use a custom format to display the day of the week of a date. To show the short name use ddd to show the full name use dddd. See the two examples below.
If you want to use the Day name as text use the TEXT function to convert it to text. The formulas used in the image below are
Note: the format cells (columns B and C) are right aligned as dates are treated as numbers. The TEXT function cells (columns E, F and G) are left aligned because the TEXT function returns a text result.
If you wanted to show just the first letter of the day eg M for Monday for a roster or a chart, you could use the following formula