A day subsequent in time to a given event by some significant period other than a year (especially as prefixed by the amount of time in question).
A day that is an exact number of years (to the day) since a given significant event occurred. Often preceded by an ordinal number indicating the number of years.
Such a day that commemorates a wedding.
one day of solar time, the time between successive noons; the time in which a planet such as Earth apparently rotates once around its axis, relative to the sun (on Earth about 24 hours as an average - the length of solar day is not constant throughout the year)