Samsung has undercut Apple with the introduction of a system-wide, dark theme with a user interface, but the ability to schedule Night Mode to be automatically enabled has not been considered. However, it is not something that can not be fixed with a small update, and that is what Samsung has done to fix this problem.
Granted, you always had the option to automatically set the night mode through the Developer Options, but the option was fairly hidden and severely limited in your scheduling options. It was also a bit flawed and could be turned on or off randomly throughout the day, as some users report.
Fortunately, Samsung has fixed this bug with a software update that makes scheduling night mode much easier ̵
Scheduling Night Mode
To schedule night mode, first make sure that you are running the latest version of One UI. The update was applied to most newer Galaxy models at the time of writing. Check in the settings, if an update is available, if you have not received one yet.
After updating, open the settings and choose View. Next, tap the "Night Mode" text on the following page (not the toggle). From there, turn on the "Turn on schedule" switch and choose between "Sunset to Sunrise" or "Custom Schedule" to your liking.
Note that if you choose from "Sunset to Sunrise" you also need to activate your phone's GPS to see your exact position relative to the Sun. For "Custom Schedule", your phone's time will be automatically turned on and off at the time you specify.