Summer solstice on June 20, 2016 will be the longest day of the year. The length of the day is measured by length of sunlight. In Chinese Medicine, we think of this day as the utmost yang of the year, which means it is the peak of yang in the yang part of the year, signified by the sun sitting at the highest point in the sky. The yang part of the year includes the spring and summer, while the fall and winter are considered yin, relatively. The measure of yin and yang are relative to one another and at their peaks, begin to transform into the other.
Simply said, think about something new growing from inside something older. A constantly evolving flow with a forward momentum.
So imagine Spring as yin within yang, while the Summer is yang within yang because summer is more yang than spring. At the summer solstice, once reaching the longest day then begin to shorten the following day. As the evenings start to grow longer, we will approach fall.
The Fall then can be seen as the yin within yang. The evenings continue to grow longer until the longest night of the year, which is the winter solstice, December 21, 2016. The peak of yin.
The summer solstice represents the maximum of yang possible, before implicit in this understanding is also that the yin begins to grow again and the yang wanes.
SPRING
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Yang within Yin
SUMMER
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Yang within Yang
FALL
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Yin within Yang