The problem has a tag "regex", hence finding a regex solution might be the better solution

:= my bad

You missed this part of the description:

During leap years February has 29 days.

I'm wondering...

days in month are as follow :
Month lengths from January to December: 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31.

The test says :
firstDate = "22/02/2020" k = 1 daysOfTheWeek = ["Saturday"] n = 2
expected = ["22/02/2020", "29/02/2020"]
Test.assertDeepEquals(recurringTask(firstDate, k, daysOfTheWeek, n) , expected)
So how do we find the expected result "29/02/2020" when february is only 28 days?

I started using Date but got stuck on a verification submission test that used 28 days instead of the real number of days for that year, but then on rerun I got stuck on the test examples ;D
I'm moving to a new exercize !