Days	Months	Seasons
Montag · Dienstag · Mittwoch	Januar · Februar · März · April	der Frühling — spring
Donnerstag · Freitag	Mai · Juni · Juli · August	der Sommer — summer
Samstag · Sonntag	September · Oktober · November · Dezember	der Herbst — autumn
		der Winter — winter

They're all masculine, and the little word in front tells you which kind of time you mean:

• am for days and parts of the day: am Montag, am Wochenende, am Abend.
• im for months and seasons: im Juli, im Sommer.
• um for clock times: um neun Uhr.
• Add -s for a habit: montags — on Mondays, abends — in the evenings.

In parts of northern and eastern Germany, Berlin included, you'll also hear Sonnabend for Saturday. And in Austria, January is Jänner.