MoonPhaseDate

MoonPhaseDate[] returns the date of the next new moon.

MoonPhaseDate[phase] returns the date of the next instance of the given phase of the Moon.

MoonPhaseDate[date,phase] returns the date for the first instance of the given phase after date.

Examples

Get the next full moon date:

MoonPhaseDate["FullMoon"]
(* DateObject[...] *)

Please visit the official Wolfram Language Reference for more details.