WeekDay and BirthDay

You know your birthday date, no problem. Do you know on which day of the week you were born? If not, you can find it out quite easily in Excel. There are many ways to do it, but probably the simplest one is to use the WEEKDAY function, as shown in this snip:

The syntax for WEEKDAY function in Excel is:

WEEKDAY(serial_number,[return_type])

where serial_number practically means a serial number (like e.g. 40232) or an entry in any acceptable DATE format, e.g. 15 Nov 2021, and the return_type can be omitted (defaults to 1). This looks OK; you get a correct day of the week in return.

However, when you try to use different return_types, things may become a bit confusing. Here is an example:

So, to avoid an erroneous result and make sure that the weekday of your birth (or whatever event) is correct, it's safer to use more reliable custom formats, like these ones: 

dd/mm/yyyy ddd     or       dd mmmm yyyy, dddd

If neded, add them to your list of custom formats in Format Cells... >Number >Custom category and then select the format for some cell(s). When you enter into the formatted cells the date like 04/12/2001 (or simply =A2, based on example provided above), you'll see the following results displayed for the two formats:
 

You can be sure now, without using the WEEKDAY function, that the day of the week the event took place was Tuesday.