There’s a bug that puts random emails from Gmail as all-day events in Calendar. It is unknown how widespread the problem is and Google has not yet responded to the reports.
Source: The Verge
It seems to mainly affect users who use the Google Calendar app; it is unknown if it also affects users on the web. In any case, it concerns the apps on iOS and Android. A workaround is to disable the option to automatically convert events from emails into events in Calendar. This can be done via the settings of the app.