You'll need a "helper" cell for this.
Assumption: data is in Cells: B2:B100 and is dates
A2: =WEEKDAY(B2,2)=1
Copy the formula down as far as you need it.
That formula will return TRUE for each column B value that is a Monday.
You could then use Data>Filter>Autofilter to select only TRUE values
OR
Assumption: data is in Cells: B2:B100 and is not Excel dates
A2: =MOD(ROW()+5,7)
copy that formula down
Adjust the 5 in the formula until the return value of 0 (zero) flags the
Mondays.
This time use Data>Filter>Autofilter to select only zero values.
Alternatively, using either formula you could select Col A,
then Edit>Find
Find What: 0 or TRUE (depending on which formula you used)
Look In: Values
Click [Find All]
Then hold down the [Ctrl] key and press A
That will actually select all of the zero or TRUE cells, respectively.
Does either method help?
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Regards,
Ron