Try Network Days function -
networkdays (start_date, end_date [, holiday])
You need to specify the holidays in networkday().
For example you want networkdays between 2006-12-18 and 2006-12-31.
If you just give this without the specific holidays you will get 10 as output.
But say you have holiday on 2006-12-25 and 2006-12-26
Then specify like this
networkdays ('2006-12-18', '2006-12-31', '2006-12-25', '2006-12-26')
and you will get answer as 8.