# In the first month

"In month one" 

# which is the month of Nisan

"Nisan" is the name of the first month of the Hebrew calendar. It is during the last part of March and the first part of April on Western calendars. 

# in the twelfth year of King Xerxes

"in year number 12 of King Xerxes" or "when King Xerxes had reigned for about twelve years"

# the month of Adar

"Adar" is the name of the twelfth and last month of the Hebrew calendar. It is during the last part of February and the first part of March on Western calendars.