Learn French: Le troisième jour, j'étais fatigué.

We use jour when counting specific days as units of time or placing them in a sequence, like with ordinal numbers (le troisième jour). If we wanted to emphasize the continuous duration of the day or the events happening throughout it, we would use journée.

English uses "on" in "on the third day". Why is there no preposition like sur before Le troisième jour?

French does not use a preposition to say "on" a specific day or date. You simply use the definite article le followed by the day or number, just like you would say le lundi for "on Monday".

Why is j'étais in the imparfait tense rather than the passé composé (j'ai été)?

The imparfait is used here because feeling tired is a physical state or condition in the past, rather than a single, completed event. It sets the scene by describing how things were at that moment in time.

Does the spelling of fatigué change depending on who is speaking?

Yes. Since fatigué acts as an adjective describing the subject (j'), it must agree in gender. If the speaker is female, an extra e is added to make it j'étais fatiguée, though the pronunciation stays exactly the same.