I've been using cron(8) for donkeys' ages, but this morning I'm surprised.
Machine was powered down since last Saturday (long story) and booted last night. I have a crontab(5) entry with day set to "*/2" and am surprised the job didn't run this morning: Dec 6th, I'd have thought */2 is even days only, but it seems it actually means "every second day of uptime". crontab(5) here doesn't really elaborate.
Until this morning I had not given that further thought.
Live and learn.