Use the Weekday function along with the DateAdd function to get Monday's date.
Public Function MonDt(Dt As Date) As Date
Dim DtIn As Date
Dim DOW As Byte
DtIn = Dt
DOW = Weekday(DtIn, vbMonday)
MonDt = DateAdd("d", -DOW + 1, Date)
If you are trying to do it as a calculated field in a query, I think it would be something like
MonDt: DateAdd("d", -WeekDay(DateField, vbMonday) + 1, DateField)