Using 'am' with days and times of day
When talking about schedules and routines, English speakers have to juggle a few different prepositions. We do things on Monday, but we do them in the morning or at the weekend.
German makes this much easier. For almost all days of the week, special dates, and parts of the day, you only need to remember one small word to pinpoint when an event happens: am. Whether you are making plans for Dienstag or scheduling a meeting for the Nachmittag, am is your go-to preposition.
The word am is actually a contraction. It is a blend of the preposition an (meaning at or on) and the dative article dem. In German, time expressions usually require the dative case.
Because the days of the week and the parts of the day are masculine words, they take the article dem in the dative case. The two words an and dem naturally squish together to form am. The word Wochenende is neuter, but neuter words also take dem in the dative case, so the rule works perfectly there too.
Whenever you would use on in English for a specific day, you will use am in German. This applies to the entire week: Montag, Dienstag, Mittwoch, Donnerstag, Freitag, Samstag (also known as Sonnabend in some regions), and Sonntag.
This rule also smoothly extends to special days and holidays, especially those ending in the word Tag. You will use am for Geburtstag, Feiertag, Neujahrstag, and Valentinstag.
This is where English speakers usually trip up. In English, we say in the morning or in the evening. You must resist the urge to translate this directly into German. Instead, stick to your new favorite time word: am.
You will use it for all the masculine daytime periods: Morgen, Vormittag, Mittag, Nachmittag, and Abend.
There is one very important exception to this rule. The German word for night is feminine, which means it cannot use the article dem. Because am is strictly for masculine and neuter words, you cannot say it with the night.
Instead, you have to memorize a different phrase. To say at night, you say in der Nacht. If you want to talk about midnight specifically, you use yet another preposition and say um Mitternacht.
When you want to say that something happens on a certain day, on a holiday, or in a specific part of the day, just use am. It consistently replaces both on and in the for all your daily scheduling, as long as you remember that the night is an exception.