Learn Finnish: Miten aikataulutat viikon, kun laulat kolmissa ristiäisissä? - How do you schedule the week when you sing at three christenings?

Because the noun ristiäiset (christening) is a "pluralia tantum" word, meaning it only exists in the plural. When you use a numeral with a plural-only noun, the numeral must also take a plural form and match the case of the noun. Since the noun is in the plural inessive (ristiäisissä), the number three becomes kolmissa.

Why use the inessive case (-ssä) to say "at christenings" rather than the adessive (-llä)?

In Finnish, many life events, ceremonies, and gatherings take the inessive case (-ssa/-ssä) to express being "at" them. You can think of it as being "inside" the event. This applies to words like ristiäiset (christening), häät (wedding, becoming häissä), and juhlat (party, becoming juhlissa).

Why is the object "the week" written as viikon instead of the partitive viikkoa?

Viikon is the total object here (which takes the genitive form). Using it with the verb aikataulutat implies a completed action: you are scheduling the entire week from start to finish. If you used the partitive viikkoa, it would mean the scheduling is an ongoing, unfinished process, or that you are only scheduling part of the week.

Is aikataulutat related to the noun for "timetable"?