Referring to the article on the calendar table, you could add a [month] column that would makes this easy for you.

You would create the field and update it by adapting the following:

select [calendardate]

, dateadd(d,-day([calendardate])+1,[calendardate])

from calendar

From here the calculation becomes a simple matter.

select[month], count(*) numWorkDays

fromCalendar

where[calendardate]>= @fromDate and [calendardate]<=@untilDate and

workday = 1

group

by [month]