The likely cause of this problem is that you're synchronizing your iPhone Calendar with iCloud or another online calendar service and your iPhone is set to sync only recent events. This option can be found underMail, Contacts, Calendars
in your iPhone Settings
app. The default setting is one month, however you can tap on this option and choose from either two weeks, one month, three months or six months or simply choose All Events
to sync everything in your calendar.
New events created in the iPhone Calendar outside of this time frame are still saved and synchronized to your online calendar (e.g. iCloud), but once the online calendar has been updated, they are removed from the local iPhone Calendar; hence these events appear to vanish shortly after you've created them. Items you have created on these older dates will actually have been saved in the cloud and you would likely be able to see them on a desktop computer; iCal for Mac, for example, automatically synchronizes all events from iCloud to your desktop calendar, regardless of the setting on your iPhone. Further, if you're using iCloud you can log onto iCloud.com from any web browser and you should see these events in your Calendar there as well. In fact, if you adjust the sync setting above to a longer time frame-or All Events
-all of these "lost" events should automatically be synced back down from the cloud and reappear; you'll likely even see all of the instances of events that you tried to create multiple times.
Ultimately, unless you have a very large calendar going back several years, it's probably best to just set your sync events preference to All Events
, which will ensure that your entire calendar is available on your device and avoid any future confusion with vanishing events.