It isn't really a spoiler, but since it is actually a plot point (...Sort of. More like a gag.) that occurs at the very end, better safe than sorry; some people are very prickly about what could possibly be considered a spoiler. Though the author does makes jokes in some of the volume notes about how the series really is just about going with the flow in an endless summer, so the timelessness is intentional, to a degree.
True with SY... if I ever go back and reread all of it, I should keep track of the holidays (and other similar events).
Though I don't think you can get much more slipshod with time than Detective Conan, especially considering that it takes place in a realistic world that utilizes technological developments. That thing's been going on for 25 years.