The day begins at sunset, based on Bereishit 1:5 "and there was evening and morning, one day". However, doing creative work (melacha) on a major holiday or on Shabbat is a violation of biblical law, so to be safe we add some time to both ends. Most communities (not all!) begin Shabbat or a holiday 18 minutes before sunset and end it about an hour after sunset the next day (full dark).
Your calendar is presumably trying to reflect this "fudge factor" -- the day begins at sunset, but the observance of the day begins a bit before, so "approximate". This is good enough for most users of that calendar and considerably better than some calendars (which simply note the day on the next secular day). Those who are observing the day know to adjust.
For some sources about the 18-minute calculation, see this answer.