Generally, Doug, dictionaries regard anniversaries as being a date on the calendar, not a moment in time. Same with birthDAY. A minute before the solstice (God, nearly said "prior to" there...), the northern hemisphere is still a fraction away from having the sun at maximum angle in the sky, and a minute after the sun is declining from that angle. The "-stice" part of the word only applies momentarily.

After today's moment in time, the sun's angle declines on a daily basis, albeit almost imperceptibly so unless observed over a week or two. Sorry, couldn't resist...