Step 1
Go to the Mes calendriers page. Click Éditer on the calendar which you want to extend.
Google Calendar shows only some events, or nothing at all.
Google Calendar has a 1 Mb limit on calendar imports. If the file is larger than this, Google will only show some events, or none at all.
Mostly this affects the Soleil calendar. It can be set to display many daily events. If the time window is long, the total file size will exceed Google's limit. One month is a safe bet, and usually 6 months works fine as well.
Step 2
Open the Période de temps tab and adjust the time periods to your liking. Try one month or at most six months.
The Période de temps tab is not shown on phones due to their small screen size. You'll need to adjust this setting with a tablet or a computer. The extended calendar, however, will work fine with phones as well.
Finally, click Sauvegarder.
Step 3
Your calendar is now small enough, but Google does not know about it. You can either wait for Google to download the updated calendar, or do it manually.
If you want to do it manually and see the results quicker, please take a look at How do I force my calendar application to refresh a calendar subscription?
Updated 8 Feb 2023