By default, the iPhone will only sync events that are not older than 2 months. If you want to keep all events, you have to change this in the System Settings of your iPhone, or in iTunes.
■iPhone Settings
1.Open System Settings
2.Go to Mail, Contacts, Calendars
3.Scroll all the way down to Sync
4.Set Sync to your preferences
■iTunes Settings
1.Connect your iPhone or iPod touch to iTunes
2.Set Do not sync events older than 30 days to your preferences