calling a function

Hi All

I want to write a function. where user enters a DATE in A1 and it shows the day in B1.

i know there is a text function in excel like text(a1,"dddd") which gives the value
but if we have to write this function how to proceed.

Also

where do we call this function
a)Private Sub Worksheet_SelectionChange(ByVal Target As Range) or
b)Private Sub Worksheet_Calculate()

Basically whats the diffrenece between this a and b

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many thanx