I'm going to work late on Saturday and ask for today off in its place.
That is short notice. Isn't it a bit late to ask for today off don't you think?
I can get today off, I just want to have fun and hangout today with my friends.
If you take today off then it's likely that your co-workers will have to work late to cover your shift. That's a bit inconsiderate and selfish and you shouldn't take sudden time off like this. You need to respect your co-workers and employer or you might be the first to get fired one day.
I just want to have fun so I'll take the day off today. I'll be out at 5pm today.
You can't just set your own hours. You need to work when you are expected just like everyone else has to.