It is very simple after understood:
- open the Calendar page
- on left notice "Other calendars"
- press "Add"
- "Browse interesting Calendars"
- "Holiday" tab is selected by default
- Click "More" tab
- on line with "Week Numbers" click "Subscribe"
- done, click "Back to calendar"
In "Week" view, it appears for the first day of the week (can be set), above the hours.
In "Month" view, it appears for each first day of the week.
If tried to search for help:
- you get this page
- but there can't be found the info
- you have to open "More"
- there you'll find the "Week Number" procedure