Use までに to express that something happens or must be done by a specific point in time. It emphasizes the limit: the action occurs at some time before that limit, not exactly at it. For example, '5時までに' means 'by 5 o'clock', so if you arrive at 4:50, that's fine; if you arrive at 5:01, it's too late.

When the attached verb is in the dictionary form, the deadline is set for that action to be completed. When it's in the past tense, the deadline is relative to that past event (e.g., '到着するまでに' = before arrival, or '到着したまでに' is less common; the dictionary form is typical). In everyday conversation, までに is common and neutral in tone, used in both casual and formal settings.

Note the difference from まで: まで indicates a continuous action or state that continues until the time point, while までに indicates a one-time action that must happen before the time point.