April is the fourth month of the year in the Gregorian Calenar, and one of four months with a length of 30 days. April was originally the second month of the Roman calendar, before January and February were added by King Numa Pompilius about 700 BC. It became the fourth month of the calendar year (the year when twelve months are displayed in order) during the time of the decemvirs about 450 BC, when it also was given 29 days. The derivation of the name (Latin Aprilis) is uncertain. The traditional etymology from the Latin aperire, "to open," in allusion to its being the season when trees and flowers begin to "open," is supported by comparison with the modern Greek use of ἁνοιξις (opening) for spring. Since most of the Roman months were named in honor of divinities, and as April was sacred to Venus, the Festum Veneris et Fortunae Virilis being held on the first day, it has been suggested that Aprilis was originally her month Aphrilis, from her Greek name Aphrodite (Aphros), or from the Etruscan name Apru. Jacob Grimm suggests the name of a hypothetical god or hero, Aper or Aprus.