I often want to know what day of the week a date is. Excel has a function for this, but unfortunately it only returns an integer, for example the function:
=WEEKDAY("01/02/2009")
returns 1 (which represents Sunday).

This can be a bit confusing with a list of dates, so I wrote the following quick VBA function to get the day of the week as text, e.g. Sunday, Monday, etc.

The function (possibly not the most elegant method possible) is:

' function to return a text representation of the weekday of a given date
' returns the full text, which can be shortened by left(), etc
Function dayText(d As Date) As String
If Weekday(d) = 1 Then dayText = "Sunday"
If Weekday(d) = 2 Then dayText = "Monday"
If Weekday(d) = 3 Then dayText = "Tuesday"
If Weekday(d) = 4 Then dayText = "Wednesday"
If Weekday(d) = 5 Then dayText = "Thursday"
If Weekday(d) = 6 Then dayText = "Friday"
If Weekday(d) = 7 Then dayText = "Saturday"
End Function

(Enter this via the VBA Editor – press Alt+F11, then insert -> module and paste this code in).

It can then be called by:
=DAYTEXT("01/02/2009")
which returns Sunday.

This can then be shortened to one or three letters via the left() function, for example:
=LEFT(DAYTEXT("01/02/2009"),3)
returns Sun.

If you want to avoid macros, an alternative method is to use the (slightly messy) formula consisting of nested IFs, for a spreadsheet with a date in cell A1:
=IF(WEEKDAY(A1)=1, "Sunday", IF(WEEKDAY(A1)=2,"Monday",IF(WEEKDAY(A1)=3, "Tuesday", IF(WEEKDAY(A1)=4, "Wednesday", IF(WEEKDAY(A1)=5, "Thursday", IF(WEEKDAY(A1)=6, "Friday", "Saturday"))))))