For the best experience, and to continue to receive all the notifications, you should open the app at least once every week.
The reminders for Rosh Chodesh, Shabbat, ‘Omer and other alerts, are implemented with iOS Notifications.
An application is able to register a certain number of scheduled notifications that the system pops up at the specified time in the future, even when the app is not open or not in the foreground. However, Apple limits the total number of scheduled notifications that a given app can add to the queue. The distance in time is not limited, but, given the number of events that CalJ must register even for a relatively short period, the app cannot schedule many Shabbat alerts in the future.
When you open the app, CalJ programs the notifications that should pop in a near future. If you open the app regularly, you give it the opportunity to keep on scheduling the upcoming alerts. If you keep the app closed for a long time, past the last scheduled notification, then there are no more reminders for you, until you reopen CalJ.