This radical was developed from a circle with a central dot as the image of the sun. As you can hardly draw circles or dots with a brush, it became the current rectangular shape.
The plants 艹 are made by the sun 日 to be large 大 [= as long shadows] at sunset. - The very similar radical stands for sunrise. This seems bizarre, but can be argued based on the characters for morning 朝 in comparison to evening 暮.
Tip: Go to the radicals that are contained in this character and learn all the characters with this radical systematically!
Vocabulary
一旦
yī dàn
in case (sth happens); if; once (sth happens, then...); when; in a short time; in one day
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通宵达旦
tōng xiāo dá dàn
overnight until daybreak (idiom); all night long; day and night
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