“Are you busy next weekend?”
Either you are, or you’re not. Females take it much more personally if a friend says she will show up and doesn’t. Plans are more concrete in our minds. Whereas men tend to make more vague plans like, “Yeah, lets try to meet up sometime next weekend.” So, even if in your head “I don’t know” is the genuine answer because you’re going to try to squeeze in a dozen friends at a dozen unnamed times, to us “I don’t know” sounds like, “I’m free but waiting to see if something better comes along.” You either are busy, or you are not. So if you’re maybe busy, you are.