Google Calendar appears to have a bug when showing public calendars when a user is logged into Google. This bug particularly affects Google Chrome users.
Different ways to solve this problem:
Follow instructions at the bottom of the Member Calendar page and add the YPSO calendar to iCal, Outlook or other calendar program -- this way you'll have access to the YPSO Member Calendar from your native calendar program and get the latest updates! Particularly useful if you have a mobile phone.
Log out of Google and reload the calendar
Open an incognito or Private Browsing window and go to the YPSO website again