first/last working day of week

If your week begins on Sunday and the first working day is Monday, then the date of the Monday in the current week is:

=CHOOSE(WEEKDAY(TODAY()),1,0,-1,-2,-3,-4,-5)+TODAY()

In the subject you use Working day and in the description you use Weekday. In Excel these are 2 different things. Working days exclude weekends and can optionally exclude any dates identified as holidays. (Need a separate list of holiday dates created in your workbook.)

Weekdays is the 7 days of the week.

If you are just looking for Monday and Friday then the following. 

 Return Monday of current week based on today's date:

=TODAY()-WEEKDAY(TODAY(),1)+2

Return Friday of current week based on today's date:

=TODAY()-WEEKDAY(TODAY(),1)+6

Caveat:

If today() is Sat then returns the date of previous Mon and Fri.

If today() is Sun then returns the following Mon and Fri.