In modern English the seasons spring, summer, autumn and winter, don't start with capital letters. However, it hasn't always been thus. For example,
1667 Milton Paradise Lost iii. 43 The..sight of vernal bloom, or Summers Rose.
or
1719 D. Defoe Life Robinson Crusoe 124 The Seasons of the Year might generally be divided, not into Summer and Winter, as in Europe; but into the Rainy Seasons, and the Dry Seasons.
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1805 Ann. Rev. 3 584 He was in the Fleet..deserted by his three Summer friends.
or
or
1868 W. Morris Earthly Paradise i. 216 When Summer brings the lily and the rose.
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1886 Leslie's Monthly Feb. 203/1 Old Sampson don't like the Summer gentry.
When did summer lose its capital?