This is the 116th most frequent German word.
samstag
"Samstag" means "Saturday" in English.
The word 'Samstags' is used in its plural form here to indicate every instance of the day, hence indicating a habitual action.
Samstags schlafe ich gerne lange.
On Saturdays, I like to sleep in.
The phrase 'nächsten Samstag' indicates a specific upcoming Saturday, modifying 'Samstag' with a future time specification.
Der Film läuft nächsten Samstag.
The movie is showing next Saturday.
Here, 'am Samstag' specifies the day on which the activity happens, thus it's used as a temporal phrase to indicate time.