Access Date Function that is the equivalent of to_char(Date,'w') in Oracle

datediff("w",date1,date2)

or....
format(Date2,"ww")-format(Date1,"ww")
this will give the number of weeks difference
(I have never used datediff to output weeks, try ww alos, let me know)

I think you want:
DateDiff("ww",[DateField1],[DateField2])

However - this doesn't work if the first two dates happen to be in different weeks (as they are for March 1st and March 2nd).

What you will need to do is write your own calculation:
WeekDifference: (([Date2]-[Date1])\7)

or something similar. I don't know how the Oracle function works so it's difficult to be more explicit. Can you describe it in more detail if the above does not fulfil your requirement ?

Use the Format function

Format(myDate,"ww")

or the DatePart function

DatePart("ww",myDate)

to get the week of the year (1-51).

There is no built-in function to calculate the week of the month, but can calculate that by subtracting today's date from the first day of the month:

Function WeekOfMonth(dtIn)

Dim dtFirst As Date
Dim btWeek as Byte

dtFirst = CDate(Month(dtIn) & "/" & Year(dtIn))
btWeek = DatePart("ww", dtIn - dtFirst)

If btWeek > 51 Then
  WeekOfMonth=1
Else
  WeekOfMonth=btWeek
End If

End Function

Maybe we did not understand what you are looking for.  is it possible you want to know that mar1~mar7 is week 1, mar 8~mar15, week 2...?   You should be able to sinply divide the day date by seven to get the week,if there is a remainder, add 1 'cause you're in the next week.