If you'd like to follow along with the small seasons calendar via iCal, you'll want to create a new calendar subscription. On Mac, you can do this by going to File > New Calendar Subscription…
Into the panel that opens, paste the following URL:
https://calendar.google.com/calendar/ical/4eu29iflrjlhahh3kusp454n2c%40group.calendar.google.com/private-3f3d86651493934f026c4aca11baaf02/basic.ics