You can always say: ‘I’m reading a book or watch series x atm, why what are you up to / why do you wanna know?’
Choose a book or series you know by heart and you always have an alibi.
If they are the kind who asks are you free on day x? Without stating a reason ask ‘why do you wanna know?’ Or say ‘I think I’ve got plans that day why?’
I don’t get it tbh, someone enlighten me please.