You can add this calendar to your own by clicking the button at the bottom-right of this calendar.
If you don’t use Google Calendar, copy this link:
https://calendar.google.com/calendar/ical/2ttvq1sfpibbugl2cnqotm7c40%40group.calendar.google.com/public/basic.ics
and for example,
- in Framagenda: use the “New Subscription” button and paste the previous link.
- in Thunderbird: “File” -> “New” -> “Calendar” -> “On the Network” -> “iCalendar(ICS)” -> paste.