The way I feel this needs to be solved is to add an invalid business days option to the time command which lists days that are not valid business days.
So you would have something like (using "B" to signify a business day shift):
{time: LLL; shift=+5B; notbusiness=SAT, SUN, 12-25}
Which would skip over Saturday, Sundays and December 25th (regardless of whether it was a Saturday or Sunday).
This would allow the user full control over what is business day and what wasn't. My concern though is how clunky this syntax is. It makes the snippet significantly more complex and also is a difficult to configure for the user.
My other concern are holidays like easter that don't fall on a fixed day each year which would require the user to continually update their list periodically.