In order to track recipients' clicks on various links that are embedded in eNotices, Finalsite appends some tracking data onto the end of each URL. Because Google Calendar links tend to be pretty long, the link can become so unwieldy when the tracking data is appended that end users see an error message when they click on it.
To get around this, use a URL shortening service such as bit.ly or goo.gl to cut that big link down to size. Just copy the Google Calendar URL and paste it into the appropriate field in one of the URL shorteners.
The result will be a much shorter link to use in your eNotice. Because the tracking data is appended onto the shortened link, you'll still be able to see which users clicked on the calendar link within your eNotice.