Breakdown of Mein Geburtstag ist am ersten Januar.
sein
to be
mein
my
der Geburtstag
the birthday
der Januar
January
Questions & Answers about Mein Geburtstag ist am ersten Januar.
Why is it ersten and not erste?
The word am puts this part of the sentence into the dative case. Ordinal numbers (like first, second, third) act just like adjectives. After a dative article (which is hidden inside am), the adjective gets an -en ending. So erste becomes ersten.
What exactly is am doing in this sentence?
am is a contraction of the preposition an (on/at) and the dative word for "the", which is dem. In German, you always use am to say something happens "on" a specific day or date.
If the number one is eins, why does first become erste?
Just like in English where "one" becomes "first", the German word for "first" is an exception. The base word is erste. Another irregular one to watch out for is "third", which is dritte. Most other numbers from 2 to 19 just add -te (or -ten after am) to the normal number.
Do we need a word for "of" before Januar?
No, German doesn't use a word for "of" in dates like English does. You just put the month right after the ordinal number. So am ersten Januar literally works like saying "on the first January".