I object to 'girls day'
Is there any reason behind it?
Yes, it is unfair I think guys should have their own day too so they can discuss their problems between each other
Having a guys days seems like a good idea.   It would be better to focus on trying to create a guys day rather than take  away a girls day.  Taking an event away from someone would ruin the day of many people.
I still don't see why girls have a day in the first place
For the same reason why you want a guys days.   It is better to focus on creating a good experience for yourself and avoid taking something away and creating a bad experience.