Here's how I use it:
“I don't want to practice now.” DO IT ANYWAY.
“I'm tired.” DO IT ANYWAY.
“It's my birthday.” DO IT ANYWAY.
“I'm just not feeling it.” DO IT ANYWAY.
“The sun is out.” DO IT ANYWAY.
“[Anything at all.]” DO IT ANYWAY.
Practicing depends on consistency. It works best if you do it every day. Why? Because what we practice is movement, and that means muscle work. Muscles learn through consistent repetition. This is not a moral judgement, or a musical judgement. It's just how muscles work. And not just repetition, but consistent repetition. Every day works best.
Practicing is also about building habits. There are habits to our playing – for example, always holding (or sitting at) the instrument a certain way. There are habits to our practicing, too, like always starting at the same time every day. Habits are built and reinforced through consistent repetition. Every day works best.
Finally: even when you enjoy practicing, it can be hard to get started each day. When you don't want to start something, any little excuse can derail you. So it's good to develop the habit of starting regardless of any excuses. The more often you say, “Do it anyway,” the more quickly it will become habitual. And the more habitual it becomes, the easier it will be.
So: DO IT ANYWAY.
PS: I understand that there are things in life more important than practicing, and that these things come up often. Practicing every day is a target that we can't always hit. But the value in a target is not in hitting it. The value of a target is in aiming at it. Yes, it's good to know whether you hit the target, but only because that information prepares you for the next time you aim at it.
DO IT ANYWAY.