birthday
The German word 'Geburtstag' is used to express the anniversary of the day someone was born, just like in English. It is often celebrated with a party, gifts, and a special cake. The word is a compound of 'Geburt' (birth) and 'Tag' (day), literally meaning 'birth-day'.
Today is my birthday.
In German, the word for 'birthday' is 'Geburtstag'. The phrase 'Heute ist mein Geburtstag.' would be used to tell someone that today is your birthday.
Your sister's birthday is tomorrow.
The phrase 'Der Geburtstag deiner Schwester ist morgen.' is a way to inform someone in German that their sister's birthday is the next day.
I forgot your birthday.
The phrase 'Ich habe Ihren Geburtstag vergessen.' is a phrase in German that one might use to apologize for forgetting someone's birthday.