Learn Italian: Sono stanco perché ho avuto una giornataccia in ufficio. - I am tired because I had a bad day at the office.

It takes the base word giornata (day) and adds the derogatory suffix -accia. This suffix gives a negative or unpleasant quality to the noun it attaches to. It is a more colorful, native-sounding way of saying una brutta giornata (a bad day).

Why does this sentence use giornata as the base word instead of giorno?

While giorno is just a calendar unit of time, giornata emphasizes the span of time and your experience during those hours. Since this sentence is about the exhausting experience of working through the whole day, giornata is the right choice to attach the suffix to.

Why is it in ufficio instead of nell'ufficio if the English translation says "at the office"?

For certain common daily locations like ufficio (office), banca (bank), and piazza (square), Italian uses the simple preposition in without adding the definite article. This is similar to how we say "at school" instead of "at the school" in English.

Could I say avevo (imperfect tense) instead of ho avuto here?