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One of Kuwahara Sensei's favourite expressions to portray in calligraphy is "every day is a good day". I've even got one myself at home. Everyone knows the saying; you hear it all the time even outside Zen circles. But what does it mean, exactly? I suspect that not everyone who says it really knows what they're talking about.
To me, every Zen saying means pretty much the same thing. Whether you say "Every day is a good day", or "How red the plum blossoms!" or even "The Buddha is three pounds of flax", you're always pointing at one and the same truth. You might as well say that every day is a bad day – at least then people wouldn't be tempted to quote you so often! The point is that the expression is not the Buddhist equivalent of "have a nice day", i.e. that you can find something good in every day. Or worse, that if you somehow live your life "buddhistically", that every day will magically become some happy, sunny and peaceful dream-world.
On the contrary, "Every day is a good day" really means that in this life, on this day and at this moment, you have absolutely no choice. "Good" in this sense therefore doesn't mean "as opposed to bad", but rather "as opposed to nothing at all", since everything else is purely the product of your own imagination. You are standing right where you are and that's the end of it: you can make choices for the future, but the present is right here and now: this is it! To take a concrete example, I'm often in a lousy mood when I get up or when I get home from work. I just want to write off the rest of the day and mope around. But Zen tells me that I only have my own means, and my own tools, to work with right now, so get to it! It's easy to think that I'm having an "off day" and that I'll feel better tomorrow, but in truth it's not some mysterious mood that's making me unhappy, but rather that very comparison to tomorrow itself. Wishing I were otherwise just makes me even more pathetic: I'm not superhuman, so that the only way that every day really becomes a good day is if I give up all theoretical comparisons with "a potential me" altogether.
Understanding this and putting it into practice are two different matters, of course. That's where Zazen comes in. And by coincidence, the expression turns out to be very handy with regards to doing Zazen itself, since most people only ever sit when they happen to be in a good mood for it. That's dangerous, however. If you want to really understand the truth of "Every day is a good day", it's better to set yourself up a weekly Zazen schedule beforehand and stick to it, regardless of "good" or "bad" mood. That, way, slowly but surely, you will come to understand the meaning of the saying. In the meantime, you can at least buy the calligraphy!