In the sentence, there are two words that define the time of the given action; “siempre” (always) and “ayer” (yesterday). Since we know the exact time when this action happened we will use the preterite tense of the verb “ir”. The subject is “yo” therefore, we will use the first person singular form “fui”.

If there wasn’t a given time in the past, we would use the imperfect. The imperfect is usually used when talking about something that happened in the past but we don’t know exactly when it started and when the action was finished.