Answer:
Full Moon
Explanation:
Full Moon does not have a fix or defined date , like some other celestial bodies .
But the rest options , i.e. , Vernal equinox occurs at March 21 every year , similarly ,
Autumnal equinox - September 22 every year
and
Summer solstice - June 21 every year ,
hence , the correct answer to statement given in the question is Full Moon .