Weekend days between dates
Over 30 days that estimate gives 8.57 weekend days, and the real answer is 8, 9 or 10 depending on which weekday you start on. Only whole weeks contribute exactly two; the leftover days contribute however many of them happen to be a Saturday or Sunday.
Counts the Saturdays and Sundays between two dates, both ends included. Over a whole number of weeks it is exactly two per week; over any other span it depends on which weekday you start on.
How to use the weekend days between dates
Every whole week contributes exactly two weekend days, and the leftover days contribute however many of them land on a Saturday or Sunday — which is why a 30-day span can hold 8, 9 or 10 of them. Estimating with days ÷ 7 × 2 gives 8.57 and is never exactly right except by coincidence. The count here walks the remainder rather than estimating.
Both ends are counted, which is the convention for date ranges and worth stating because the alternative is equally defensible for timestamps. A range from Saturday to Sunday is two weekend days here, not one.
What counts as a weekend is not universal, and this uses Saturday and Sunday. In much of the Middle East the weekend is Friday and Saturday — Egypt, Saudi Arabia, the UAE and others — and Israel’s is Friday and Saturday too. Nepal has a one-day weekend on Saturday. If you are counting for a project spanning those regions, this figure is wrong for the local calendar and the business-day calculator with a custom holiday count is the closer tool.
The usual reason to want weekend days specifically rather than working days is cost: weekend rates for staffing, kennels, car hire and hotel stays are often different, and the count you need is the number of weekend nights rather than the number of working days you are losing.
Questions
That is only right for whole weeks. A 30-day span holds 8, 9 or 10 weekend days depending on the start day.