How to recover a lost event in old iPhone's calendar?

I gave my old iPhone 4s to my little sister since I got myself an iPhone 5s. I forgot to backup my phone and my sister formatted it, and due to that I've lost my events. I did NOT turn on calendar for iCloud too.

Basically, I'm trying to find an event that I've submitted before in my iPhone 4s, this is what I got:

- Few backups from old phones
- My iPhone 5s
- My old iPhone 4s

What can I do/use to find the date of that event?

Hi, if you have a backup file, you can use FoneLab to recover calendar from iTunes backup.