Dear Dianne,
 
The lunar month is not exactly 28 days, it includes a small fraction - as a result the new moon will drift wrt the day of the week on which it appears. Genesis tells us clearly that G-d rested on the 7th day - so we are to count for the Sabbath in days, and not in lunar cycles / 4. If you count in days, then the Sabbath is always on the same day of the week.
 
in the blood of Jesus,
Lester