If you are one of them, you come to the right place.
This post fromMiniToolintroduces how to fix the issue.
The following part introduces how to fix the Windows 11 Calendar not opening on the second monitor issue.

TypeTask Managerin theSearchbox to open it.
FindWindows Explorerand right-click it to chooseRestart.
Here is how to do that:
1.

Press theWindows+Ikeys together to openSettings.
Go toSystem>Display.
verify that both screens are set to extend the display and not duplicate it.

This will ensure that both screens are treated as separate entities by the operating system.
Also see:
Way 3: Check Taskbar parameters
1.
Right-poke the taskbar to chooseTaskbar tweaks.
Expand theTaskbar behaviorspart and check theShow my taskbar on all displayoption.
Way 4: Update the Graphics Driver
Outdated graphics card drivers can sometimes cause display-related issues.
Then, you may encounter the Windows 11 Calendar not opening on the second monitor issue.
Thus, you should update the graphics driver.
OpenDevice Managerby searching for it in theSearchbox.
Right-click your graphics machine and selectUpdate driver.
Then, you might see two options Search automatically for driversandBrowse my computer for drivers.
you’re able to choose one of them.
Its recommended to back up your important files in case of unexpected things happen.
To do that,Windows backup software MiniTool ShadowMaker is suitable.
This tool is compatible with Windows 11/10/8/7 all versions.
you gotta press theWindows+Fkeys together to openFeedback Huband key in your feedback.
I hope that this post will be useful to you.