I believe it might be more complicated than some cases...

I would probably set up a table "tblFiscalYear"
with 2 fields

Yr                      Long
FiscalYearStart  Date/Time

enter as many Yr's as appropriate,
then run an update query that calcs
the fiscal year start.....

{limited testing in Immediate Window...}

yr=2005
?DateSerial(yr,4,Choose(WeekDay(DateSerial(yr,4,1)),2,8,7,6,5,4,3))
4/4/2005
yr=2006
?DateSerial(yr,4,Choose(WeekDay(DateSerial(yr,4,1)),2,8,7,6,5,4,3))
4/3/2006
yr=2004
?DateSerial(yr,4,Choose(WeekDay(DateSerial(yr,4,1)),2,8,7,6,5,4,3))
4/5/2004
yr=2007
?DateSerial(yr,4,Choose(WeekDay(DateSerial(yr,4,1)),2,8,7,6,5,4,3))
4/2/2007

then, in my query, for every date field (say "datefield")
in some table (say "sometable"), one could find the
FiscalYearStart in a correlated subquery.....

(SELECT
 Max(t.FiscalYearStart)
 FROM
 tblFiscal As t
 WHERE
  t.FiscalYearStart <= sometable.datefield) As FYStart

then, within that query (having found "FYStart"),
can you then not compute fiscal week with something
like

DateDiff('w', [FYStart], [datefield], vbMonday) + 1

This might be a case where it pays in the long run
to create a calendar table for all "possible dates"
(using Excel is touted as the easiest route, then import),

then, add fields for FYStart and FYWk

then, run update query to fill in those 2 fields
using above calcs....

truly, this would be more efficient if possible...