How do I get rid of bright line on screen that appears in calendar monthly view?

This is certainly something new - or at least, not something of which I have seen previous reports.

I would initially suggest that you for force-close the Calendar App, then relaunch Calendar:

Quit and reopen an app on iPad - Apple Support

Force-closing the Calendar will force the App to restart, instead of suspending the App. This may be enough to correct the problem. If you continue to have difficulties, a forced-restart is the next step:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Can you provide a screenshot? If you do, be careful to avoid showing any personal or sensitive information. If necessary, use the markup tools to obscure anything that you don’t wish to show.

Take a screenshot on your iPad – Apple Support

You can attach your screenshot using this button at the bottom of the editor window: