If automatic subscription does not work, you can subscribe to a calendar manually.
On the Ημερολόγια page, select a calendar you like.
Click Επιλέγω.
If you are not logged in to WebCal.Guru, you will not be able to modify subscription settings and you will have significantly smaller selection of calendars. In this case, jump directly to Step 4.
Adjust the subscription settings if needed and click Προσθήκη στα ημερολόγιά μου.
You will be taken to Τα ημερολόγια μου page. On the calendar you just added, click next to Λήψη and select Αντιγραφή διεύθυνσης στο πρόχειρο.
Open the Edit menu and select New Calendar Subscription.
Click the Calendar URL field and paste the calendar address from the clipboard.
Click Subscribe.
Usually the default settings are fine. Just make sure Auto-refresh is set, for example Every day or Every week. Usually the Location should be iCloud, but On My Mac also works fine. Click OK.
Updated 8 Feb 2023