Here are the days of the week:

Montag Monday
Dienstag Tuesday
Mittwoch Wednesday
Donnerstag Thursday
Freitag Friday
Samstag Saturday
Sonntag Sunday

Just like other nouns in German, the days of the week begin with a capital letter.

To say on what day, you use am followed by the day of the week.

If the date is mentioned after the day of the week, then there is a dem before the day of the month.

Am fünften Juli On the fifth of July
Am Sonntag, dem fünften Juli On Sunday the fifth of July
Am elften Oktober On the eleventh of October
Am Mittwoch, dem elften Oktober On Wednesday the eleventh of October

When you talk about events that happen on regular or recurring dates, e.g. on Sundays, on Mondays, add -s to any day of the week. The day will now start with a lower case letter.

Dienstag Tuesday
Ich arbeite dienstags. I work on Tuesdays.
Freitag Friday
Ich esse freitags Fisch. I eat fish on Fridays.