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Here you will see that the June solstice is drifting slightly earlier each year:
http://scienceworld.wolfram.com/astronomy/SummerSolstice.html
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The below info is cobbled together [cut'n'paste] from various sources. I'm not sure what happens over very long periods regarding the solstice dates. It depends on what adjustments are made to the Gregorian calender which will be out of wack in 5,000 years
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The Julian calendar consisted of cycles of three 365-day years followed by a 366-day leap year. Around 9 BC, it was found that the priests in charge of computing the calendar had been adding leap years every three years instead of the four decreed by Caesar. As a result of this error, no more leap years were added until 8 AD. Leap years were therefore 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, 8 AD, 12 AD, and every fourth year thereafter. What a mess !
The Gregorian calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for the Earth to complete one orbit around the Sun. The tropical year is not exactly divisible by the day. It's a modification of the Julian calendar in which leap years are omitted in years divisible by 100 but not divisible by 400.
The switch to the Gregorian started in 1582 (the year of the switch varying from country to country) because the 10 day difference between the actual time of year and traditional time of year on which calendrical events occurred became intolerable.
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For your own interest you may want to google "Iranian calendar" which is a more accurate version of the Gregorian calendar. when Jalal-ed-din Malek Shah Seljuq commissioned a panel of scientists in 1074-1079 AD to create a calendar more accurate than those in use at the time. Prominent among the scientists was Omar Khayyám [of The Rubaiyat fame].
In 141,000 years, the Iranian calendar will deviate from the solar cycle by one day. That, however, is far less than the Gregorian calendar's deviation, which will be one day in 5025 years.
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Michael
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