Learn Spanish (Latin America): Hoy improvisamos la expedición porque el día era hermoso. - Today we improvised the expedition because the day was beautiful.

In Latin American Spanish, finished actions take the preterite, even if they happened on the same day you are speaking. While you might learn that hoy triggers the present perfect in Spain (hoy hemos improvisado), Latin Americans strictly prefer the preterite (hoy improvisamos) for things that are already done and over with.

Wait, isn't improvisamos also the present tense for 'we improvise'?

Yes, it is! For regular -ar and -ir verbs, the nosotros (we) form is spelled and pronounced exactly the same in both the present and the preterite tenses. You have to look at the rest of the sentence to figure out the timeframe. Here, the past tense verb era (was) is your context clue that improvisamos means 'we improvised' rather than 'we improvise'.

Why do we use era instead of fue for 'the day was beautiful'?

You use the imperfect tense (era) here because you are describing the background setting. When talking about the past, use the preterite (fue) for completed events and the imperfect (era) to describe how things were, like the weather, time, or ongoing states. Since 'the day was beautiful' is a description of the conditions, it takes the imperfect.