found a ‘solution’ in the file /home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js just commented the default action and now I can see the calendar :astonished_face:

                        case 'BYEASTER':
                            options.byeaster = Number(value);
                            break;
                        case '':
                            break;
                        default:
            //                throw new Error("Unknown RRULE property '" + key + "'");
            ```

Re: Calendar failed to load.
in the solution provided by @sdetweil worked wonderfully on old versions of MagicMirror, but in a brand new instalation I got the same error but does not work at all adding the case ‘’: to the file /home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js

Any suggestions? 🙂

[2020-10-02 01:54:04.535] [INFO]   Calendar-Fetcher: Broadcasting 0 events.
[2020-10-02 01:54:08.276] [ERROR]  Whoops! There was an uncaught exception...
[2020-10-02 01:54:08.277] [ERROR]  Error: Unknown RRULE property 'RRULE'
    at /home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js:1191:23
    at Array.forEach (<anonymous>)
    at parseRrule (/home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js:1149:11)
    at parseLine (/home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js:1138:20)
    at Array.map (<anonymous>)
    at Function.parseString (/home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js:1110:41)
    at Function.RRule.fromString (/home/pi/MagicMirror/node_modules/node-ical/node_modules/rrule/dist/es5/rrule.js:2135:32)
    at Object.END (/home/pi/MagicMirror/node_modules/node-ical/ical.js:446:30)
    at Object.handleObject (/home/pi/MagicMirror/node_modules/node-ical/ical.js:483:39)
    at Object.parseLines (/home/pi/MagicMirror/node_modules/node-ical/ical.js:534:18)
[2020-10-02 01:54:08.280] [ERROR]  MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
[2020-10-02 01:54:08.281] [ERROR]  If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues