I am trying to use my calendar save notes on particular dates. However, when is post to past dates the note disappears a few seconds later. Any insight?

No. You cant do this natively as far as I'm aware. But you could look at getting another calendar / organizer application to do this.

Sent from the Droid... The one you were looking for...

Do you mean in the native calendar app, you want to create notes in days past? If so, I've done that before for things I wanted to note after the fact, but I've only ever left short notes in the notes field of an event that I added after a date had passed.

Go to Settings > Mail, Contacts, Calendars, scroll to the Calendars section and make sure you have the Sync set back further than you want these notes to be kept; or All Events if you never want them to go away.

This only affects the events on the iPad. If you are syncing with an online service it may also have limits to how far back your events are kept.