You can start with the function below.
datLastWeekday = ISO_DateOfWeek(intYearOfYourWeek, bytWeeknumber, vbSunday)
Now find the day of that date:
bytDay = Day(datLastWeekday)
If this is larger than 3, your month is the month of datLastWeekday.
If not, your month is the previous month.
bytMonth = Month(DateAdd("m", Abs(Day(datLastWeekday) > 3) - 1, datLastWeekday))
Note that if you are not using the ISO 8601 numbering method for weeks you are in trouble.
Public Function ISO_DateOfWeek( _
ByVal intYear As Integer, _
ByVal bytWeek As Byte, _
Optional ByVal bytWeekday As Byte = vbMonday) _
' Calculates date of requested weekday in a week of
' a year according to ISO 8601:1988 standard.
' Notes: Years less than 100 will be handled as
' two-digit years of our current year frame.
' Years less than zero returns a zero date.
' A weeknumber of zero returns the requested
' weekday of the week before week 1.
' 2000-12-17. Cactus Data ApS, Gustav Brock.
' The fourth of January is always included in
' the first week of year intYear.
Const cbytDayOfFirstWeek As Byte = 4
' Number of days in a week.
Const cbytDaysOfWeek As Byte = 7
' Month of January.
Const cbytJanuary As Byte = 1
Dim datDateOfFirstWeek As Date
Dim intISOMonday As Integer
Dim intISOWeekday As Integer
Dim intWeekdayOffset As Integer
' No specific error handling.
On Error Resume Next
If intYear > 0 Then
' Weekday of Monday.
intISOMonday = WeekDay(vbMonday, vbMonday)
' Date of fourth of January in year intYear.
datDateOfFirstWeek = DateSerial(intYear, cbytJanuary, cbytDayOfFirstWeek)
' Weekday of fourth of January in year intYear.
intISOWeekday = WeekDay(datDateOfFirstWeek, vbMonday)
' Calculate offset from Monday in first week of year intYear.
intWeekdayOffset = intISOMonday - intISOWeekday
' Weekday of requested weekday.
intISOWeekday = WeekDay(bytWeekday, vbMonday)
' Calculate offset from requested weekday in first week of year intYear.
intWeekdayOffset = intWeekdayOffset + intISOWeekday - intISOMonday
' Date of requested weekday in first week of year intYear.
datDateOfFirstWeek = DateAdd("d", intWeekdayOffset, datDateOfFirstWeek)
' Date of requested weekday in requested week of year intYear.
datDateOfFirstWeek = DateAdd("ww", bytWeek - 1, datDateOfFirstWeek)
ISO_DateOfWeek = datDateOfFirstWeek