The compiler of the Mumonkan, Wu-men (1183-1260), ended each of the koans in the collection with a capping verse. Most of these are as steep as the koans, but this one is very sweet --
Spring comes with flowers, autumn with the moon,
summer with breeze, winter with snow,
When idle concerns don't hang in your mind,
that is your best season.
That is Robert Aitken Roshi's translation. Aitken said the entire koan is about how to enjoy your best season, which is your ordinary mind.
This "best season" is not about blotting out unpleasant things and just living in a happy place. Zenkei Shibayama Roshi pointed out that spring flowers fall, the autumn moon can be obscured by clouds, summer can be uncomfortably hot and winter can be bitterly cold. "In fact, ordinary mind is in the vortexes of good and evil, beautiful and ugly, happy and sad, joy and pain," he wrote.
These things are all dualities, of course. Getting back the Taoist roots of Zen, I am reminded of the second verse of the Tao Teh Ching (John Wu translation) --
When all the world recognizes beauty as beauty, this in itself is ugliness.
When all the world recognizes good as good, this in itself is evil.
Indeed, the hidden and the manifest give birth to each other.
Difficult and easy complement each other.
Long and short exhibit each other.
High and low set measure to each other.
Voice and sound harmonize each other.
Back and front follow each other.
In other words, these dualities define each other and create each other. That's why ignoring the "bad" and accepting only the "good" is not the Tao, is not ordinary mind.
(The Tao Teh Ching was written a few centuries before Buddhism got to China, notice. I read an essay a long time ago arguing that Taoism must have been influenced by Indian Vedanta, which would mean that it shares some common roots with Buddhism. I don't know what current scholarship says about this, though.
And by the way, if you want an example of why it's a good idea not to get too attached to just one translation of ancient texts, see the Dwight Goddard translation of the second verse of the Tao Teh Ching -- "When every one recognizes beauty to be only a masquerade, then it is simply ugliness. In the same way goodness, if it is not sincere, is not goodness." He didn't get it at all. I'm told that the very ancient Chinese of the original text is just about impossible to render into sensible English, so the translator takes what he thinks the original is trying to say and expresses it mostly in his own words. Translations, um, vary.)
Another of the great Tang Dynasty Chan masters, Yunmen Wenyan (also called Unmon; 862 or 864-949), used to say "Every day is a good day." Zen teacher Nonin Chowaney said, "What determines this? The mind that dwells nowhere; the mind that accepts everything. This is nirvana."
And this takes us back to the beginning of the koan, where Nanquan said, "Ordinary mind is the Tao." Or, translated more literally, "Everyday mind as it is without any discrimination is Tao." Or, the mind that accepts everything without discrimination is the Tao.
One more reference, to a very old Chinese text called the Hsin Hsin Ming, or Verses in the Faith-Mind. It begins,
The Great Way is not difficult for those who have no preferences. When love and hate are both absent everything becomes clear and undisguised. Make the smallest distinction, however, and heaven and earth are set infinitely apart.
If you wish to see the truth then hold no opinions for or against anything. To set up what you like against what you dislike is the disease of the mind.
This text is attributed to Jianzhi Sengcan (also called Chien-chih Seng-ts'an or Kanchi Sosan, d. 606), remembered as the Third Patrarch of Zen. There's some question about whether Sengcan was the actual author, but whoever wrote it, the Hsin Hsin Ming remains one of the most well-known Zen texts.
Sometimes we do make choices. We will choose fresh food over moldy food, for example, and walk on a paved path rather than broken glass. But when life sends you moldy food and broken glass, metaphorically speaking, accept it. No anger or blame or self-pity; no judgments about good or bad luck. Then that is your best season.