Lets say that the day they both take off is day 0. The next day Sandy takes off will be day 4, and Morgans will be day 10. Sandy will take off 4, 8, 12, 16, and 20. Morgan will take off 10, 20, 30, 40, and 50. Both Morgan and Sandy take off day 20, so the next time they have a day off together will be in 20 days.
The interval for each time they take the day off would be every 20 days.