How to find first non-vacation day

I have a table of vacations

create table vacation (
id integer primary key,
dstart date,
dend date );


I need to find first non-vacation day before given date.

This can be done using the following procedural vfp code

function nonvacation( dbefore )

for i=dbefore to  date(1960,1,1) step -1
  select vacation
  locate for between( i, dstart, dend )
  if not found()
    return i
    endif
  endfor
return null

but this is very slow

How to implement this as sql select statement ?

Andrus.