Translation:My uncle and my father were drinking orange juice that day.
I think I may be starting to get the distinction between these tenses in Spanish. If I say "bebieron", it would be in a sentence like: "They drank orange juice, ate their breakfast and went out to chop wood". The drinking of the orange juice was all finished and they went on to other things. However, if I say "bebían" it would be in a sentence like: "They were drinking orange juice that day when in comes a gorilla." Does this make sense or do I have more work to do to understand this? Thanks.
You know, hesolomon, I actually find the correct use of the imperfect vs. the preterite to be more subtle than the correct choice of ser or estar. And there are a few verbs whose meaning actually change when used in each tense (saber is the one I know, but I bet there are a few more). But as Jamesw points out below the addition of ese día is important.