Is it necessary to perform ghusl on the day of 'Umrah, or is it permissible to do so the day before?

Washing (ghusl) before assuming the state of ihram is a sunnah. Whoever performs it will be rewarded for it, and if one omits it, there is no sin upon them, because ghusl is not a condition for the validity of Hajj or Umrah. The sunnah status of ghusl is indicated by the action of the Prophet (peace and blessings of Allah be upon him) and his command to Asma bint Umays to perform ghusl when she intended to assume ihram.