Calculate end date from start date and days per week data - algorithm

Question

I have the following variables
INPUT
start_date = 1 JUN 2020
session_days = [Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday]
sessions_per_week_count = 7 # from above data
total sesions = 20
OUTPUT
20 JUN 2020
from the above data, we can calculate that end_date should be 20 Jun 2018
I need help writing algorithm/formula to calculate the end date with variable input data based on the calender.
any suggestions will be appreciated.

Answer 1

Finally, I came up with a pretty simple method in ruby
def session_dates(date, weekdays_arr, number_of_working_days)
result = []
while result.size < number_of_working_days
if weekdays_arr.include? date.wday
result << date
end
date += 1
end
result
end
by calling a method like this
start_date = Date.today.beginning_of_month
session_days_in_week = [0,1,2,3,4,5,6] # week starts from 0(Sunday)
result = session_dates(start_date, session_days_in_week, 20)
I can get all dates on the weekdays in output
[
[ 0] Mon, 01 Jun 2020,
[ 1] Tue, 02 Jun 2020,
[ 2] Wed, 03 Jun 2020,
[ 3] Thu, 04 Jun 2020,
[ 4] Fri, 05 Jun 2020,
[ 5] Sat, 06 Jun 2020,
[ 6] Sun, 07 Jun 2020,
[ 7] Mon, 08 Jun 2020,
[ 8] Tue, 09 Jun 2020,
[ 9] Wed, 10 Jun 2020,
[10] Thu, 11 Jun 2020,
[11] Fri, 12 Jun 2020,
[12] Sat, 13 Jun 2020,
[13] Sun, 14 Jun 2020,
[14] Mon, 15 Jun 2020,
[15] Tue, 16 Jun 2020,
[16] Wed, 17 Jun 2020,
[17] Thu, 18 Jun 2020,
[18] Fri, 19 Jun 2020,
[19] Sat, 20 Jun 2020
]
and obviously the last element in result array is ending_date.