free and easy night
An evening/night where you choose to relax at your own leisure, doing whatever you want to do for your pleasure, without giving a f**k about anything else.
Example:
Girlfriend: You coming round tonight to watch a romantic film with me? Boyfriend: No, it's a free and easy night for me.
Girlfriend: You coming round tonight to watch a romantic film with me? Boyfriend: No, it's a free and easy night for me.