th - century ▁Islam ic ▁calendar ▁opt ed ▁for ▁a ▁purely ▁lun ar ▁one . ▁ ▁A ▁continu ing ▁lun is olar ▁calendar ▁is ▁the ▁Heb rew ▁calendar . ▁Ev idence ▁of ▁this ▁is ▁noted ▁in ▁the ▁dates ▁of ▁Pass over ▁and ▁E aster ▁in ▁J uda ism ▁and ▁Christian ity , ▁respectively . ▁Pass over ▁falls ▁on ▁the ▁full ▁moon ▁on ▁ 1 5 ▁N isan ▁of ▁the ▁Heb rew ▁calendar . ▁The ▁date ▁of ▁the ▁Jewish ▁Ros h ▁Hash ana ▁and ▁Su kk ot ▁festiv als ▁along ▁with ▁all ▁other ▁Jewish ▁hol id ays ▁are ▁dependent ▁on ▁the ▁dates ▁of ▁the ▁new ▁mo ons . ▁ ▁Inter cal ary ▁months ▁ ▁In ▁lun is olar ▁cal end ars , ▁an ▁inter cal ary ▁month ▁occurs ▁seven ▁times ▁in ▁the ▁ 1 9 ▁years ▁of ▁the ▁Met onic ▁cycle , ▁or ▁on ▁average ▁every ▁ 2 . 7 ▁years ▁( 1 9 / 7 ). ▁In ▁the ▁Heb rew ▁Calendar ▁this ▁is ▁noted ▁with ▁a ▁periodic ▁extra ▁month ▁of ▁Ad ar ▁in ▁the ▁early ▁spring . ▁ ▁Blue ▁moon ▁ ▁In ▁the ▁modern ▁system ▁of ▁" trad itional " ▁full ▁moon ▁names ▁tied ▁to ▁the ▁sol st ice ▁and ▁equ ino x ▁points , ▁a ▁super numer ary ▁full ▁moon ▁in ▁such ▁a ▁period ▁is ▁called ▁a ▁blue ▁moon . ▁The ▁term ▁" blue ▁moon " ▁used ▁in ▁this ▁sense ▁may ▁date ▁to ▁as ▁early ▁as ▁the ▁ 1 6 th ▁century , ▁but ▁it ▁became ▁well - known ▁in ▁the ▁United