This pattern is a formal way to say 'when' or 'at the time of'. It is commonly used in written announcements, business settings, or polite spoken Japanese. For example, you might hear it at a train station or in an email from a company. Unlike the more casual とき, 際に sounds more official and careful. It often implies a specific, important moment rather than a general situation. You can use it with verbs in their dictionary form to talk about an action, or with nouns plus の to talk about an event or occasion. Note that the verb is always in its dictionary form, not past tense, even if the event happened in the past. For instance, you say 行く際に, not 行った際に, when meaning 'at the time of going'.