Date Logic calculation

Help
1

Please help me on below logic:

Logic 1:
if (Sys date = 1st working day of Month) then {Boek Datum = Last day of Previous Month}
Else {Boek Datum = Todays Date.}

Date is entered in the 3 fields in DD / MM/ YYYY format.
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Logic 2:
If {System Date = 1st working day of Month then Boek Periode} then {Boek Periode = Previous Month and Current Year in MM YYYY Format.}
Else {Boek Periode = current Month and Year in MM YYYY Format.}
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Logic 3:
This always has a value of SysDate() - 1. Format is DD MM YYYY
Value is always entered by system. No changes allowed."