Write SQL query to solve given problem: When did event number 2 happen and how many users were active?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT COUNT(T1.app_id) AS num FROM app_events AS T1 INNER JOIN events AS T2 ON T1.event_id = T2.event_id WHERE T1.event_id = 2 AND T1.is_active = 1 GROUP BY T2.timestamp