In the background, iPhone displays a Calendar screen, showing one day’s events in portrait orientation; in the foreground, iPhone is rotated to landscape orientation, which displays the Calendar events for the whole week containing the same day.

One slight movement can cause your iPhone's screen to rotate. This can be really frustrating as you immediately lose the point where you were in the app. The fix might be easier than you expect.

If you lock the screen orientation, it will not change when you rotate iPhone.

Open Control Center, then tap the following icon > the Lock Orientation button.

If the screen orientation is locked, the icon > the Lock Orientation icon appears in the status bar (on supported models).

No rocket science here, hope it helps :)