I have seen in other discussions how to calculate the next delivery day using:

StartDate + Int ( WorkDays / 5 ) * 7 + Middle ( "12345012340123601256014560345623456" ; ( DayOfWeek ( StartDate ) - 1 ) * 5 + Mod ( WorkDays ; 5 ) + 1 ; 1 )

But in my case I need a function which includes the Start Date as well as the End Date. This is for calculating Class dates.

In other words, if a class starts on a Monday, May 8th, and runs for 10 days, the calculation shows it ending on Friday, May 26th, using a 5 days on and two days off

When trying the above calculation it shows the following Monday as the End Date. Which would be correct if I didn't need to include the Start Date as the first day.

How do I change the above calculation so it includes the Start Date? I've tried a bunch of things but just can't get it to work.

Are you sure that you want to use that formula? What happens if there is a holiday in that date interval?

There is another option that works from a table of "class dates" so that managing holidays is simply a data entry task and does not require redefining a calculation. While some use such a table in combination with a formula like what you've posted, I prefer to treat weekends as holidays and just work completely from the set of class date records in the table as that produces a simpler calculation.