The dentist you don't have
You may be thinking that you do say no sometimes. You have a way of doing it, which is to be busy.
"Oh, I'd have loved to, but I've got a thing that day." "Work's mad at the moment." "I think we've got family coming." Sometimes the thing, the madness and the family exist. Often they're invented on the spot, because a no with an excuse attached feels so much safer than a bare one. The excuse tells them it isn't that you don't want to. It's that you can't.
I understand the appeal, and I think the invented excuse is a poor tool, for three reasons. Two are mine and one is a finding.
The first is that it invites a solution. If the obstacle is the date, a helpful person moves the date. "Oh, that's fine, we could do the Sunday instead." Now you need a second excuse, and the second one is always weaker than the first. People who rely on excuses end up saying yes at the third attempt, worn down by their own story.
The second is upkeep. You have to remember the dentist you don't have. You have to not be seen in the garden that afternoon. It's a small strain each time, but it adds up, and being caught out does far more damage than a no would have done.
The third is that the most popular excuse doesn't work as well as people assume. Grant Donnelly, Anne Wilson, Ashley Whillans and Michael Norton ran six studies on the excuses people give when turning down invitations. They compared "I don't have time" with "I don't have the money". The time excuse came off worse. People hearing it judged it less trustworthy and felt less close to the person afterwards, and they were less willing to help that person later. The reason the researchers found was control. Everyone knows that time is something you allocate. "I don't have time" is heard, correctly, as "I've chosen to spend my time on something else." The people giving the excuse didn't see this coming. They expected "I'm too busy" to land softly, and it didn't.
I wouldn't take from this that you should plead poverty. The point is that the vague busy excuse, the one that feels like the polite choice, is heard as a slightly dishonest no. You pay the cost of a refusal and also the cost of a fib.
There's a related habit, which is the no that never gets said. "Maybe." "I'll try." "Let me see how the week goes." Or the message left unanswered in the hope that the question goes away. Gili Freedman, Kipling Williams and Jennifer Beer have written about this from the point of view of the person doing the rejecting. Drawing together research from several fields, they argue that a clear, explicit no hurts the other person less than an ambiguous one or silence, and leaves the person who gave it better off too. I should be plain that theirs is a theory paper. It proposes this and doesn't test it. But it fits what it's like to be on the receiving end. You can do something with a no. A maybe leaves you unable to ask anybody else.
So I'd suggest giving up the invented excuse and the maybe along with the unwilling yes. They're the same habit, which is avoiding the four seconds.
What goes in their place is simpler than you'd think. You don't owe anyone a reason. "I can't do that one, I'm sorry" is a complete answer, and most people accept it without a follow-up. If you'd like to give a reason, give a true one and keep it short. "I've had a run of busy weekends and I need that one to myself" is true, it's about you, and it can't be solved by moving the date. When Donnelly and his colleagues had people stress that their time wasn't under their control, the time excuse did less harm. I'd read that as support for being specific and honest. "I'm working the next three Saturdays and this is my one day off" tells her something real. "I'm just so busy" doesn't.