This question has come up quite frequently, so searching is really the best bet. 
By far the easiest way to do this is with a calendar table that marks weekend dates (and potentially holiday dates as well) and lets you work that way. If you canât do that, you will have to resort to some tricky math. It helps if you draw a picture or look at a calendar while youâre evaluating the formulas.
First the simple part: there are 28 days between your two dates, so you are guaranteed to have at least 4 weeks. But how many weekends can you count? The easy answer is âfourâ which would mean you have 20 business days between your two dates. Suppose today is Wednesday and you go back 28 days, you will get 8 weekend dates in that range. Now suppose that today is Sunday, and you go back 28 days, how many weekends do you get? Again, you get 8. What if today is Friday? Still 8 days. Okay, so with a range of 28 days you always get two weekend dates. Now to make it more general: if you have any date range with an even multiple of 7 days, you can count on having exactly an even number of weekend dates.
Assume âXâ is the number of days between two dates, what does this do?
truncate(X / 7, 0)It takes the number of days, divides by 7, and returns an integer result. If X < 7 you get zero. If X between 7 and 13 you get 1. If X is 14 you get 2. Why does this matter? Because you can multiply that number by the number of weeks and be guaranteed to get no fewer weekend days than that.
Why no fewer? Suppose you have a range of five days, Mon - Fri. Doing truncate (5 / 7, 0) = 0 and you take 0 * 2 and get 0 weekend days. Thatâs correct. If you have a range of 28 days, then truncate (28 / 7) * 2 = 8, which matches the answer given above.
But⌠what if the five days are Friday through the following Tuesday? Thatâs only five days, yet it includes weekend days that have to be removed. That means you have to know the start day as well as the total number of days in order to calculate properly. Still making sense? 
Itâs for this reason that most formulas you see to do this are quite complex. You mentioned seeing some other formulas here on BOB. I suggest you go back and look at them while looking at a calendar and see if you can figure out how they work. If not, post a specific question and see if someone can help clear things up.
Or get a weekend flag in your calendar table.
[Edit] A bit of searching got me to this post which provides some of the same details I did but finishes them out more, I think.
Dave Rathbun 
(BOB member since 2002-06-06)