One way round NLS settings could be
mod(trunc(mydate)-to_date('01-01-1900','DD-MM-YYYY'),7) not in (5,6)
It's a fudge, and won't work for dates before 1900. No doubt someone can dress it up . . . .

pando's code is more transparent and works for all dates. It may also be neat start for other definitions of a weekend (e.g. Muslim Thu & Fri).