Should I use preterit or imperfect to express something that used to happen repeatedly?

Question

For example, if I wanted to say "They used to travel every day", which would I use:

Ellos viajaron cada día.

Ellos viajaban cada día.

Answer 1

Imperfect, always.

That is the correct tense to use whenever you have an habitual action in the past.

Edit:

As César mentioned, a possible literal translation of your example would be

Ellos solían viajar todos los días

Whether you use solían, and cada día vs. todos los días will depend on how the sentence continues and what is the main point you want to stress (if it's the traveling, the fact that it was done every day, or something else that happened every day that caused them to travel)