N.B. Samstag is the word used to translate "Saturday" in southern Germany, wheras Sonnabend is used in certain parts of northern Germany (including Berlin).
On Monday(s)
To translate "on Monday" etc., you should precede the day of the week by "am" - i.e. am Montag, am Dienstag, am Mittwoch etc.
This is because all days of the week are masculine. Most of them are compound nouns whose final element is der Tag, the German word for "day", but the other two - Mittwoch and Sonnabend - are also masculine.
There are specific adverbs to translate "on Mondays" etc., when this implies "every Monday". Note that the fact that these words do not start with a capital letter:
Notable days in the calendar
A number of notable days in the calendar are compounds of the names of German days of the week.They are therefore used the same way grammatically - "am Karfreitag" means "on Good Friday":
