Eleazar ben Killir, also known as Eleazar Kalir, Eleazar Qalir or El'azar HaKalir (c. 570 – c. 640) was a Hebrew poet whose classical liturgical verses, known as piyut, have continued to be sung through the centuries during significant religious services, including those on Tisha b'Av and on the sabbath after a wedding. He was one of Judaism's earliest and most prolific of the paytanim, Hebrew liturgical poets. He wrote piyutim for all the main Jewish festivals, for special Sabbaths, for weekdays of festive character, and for the fasts. Many of his hymns have found their way into festive prayers of the Ashkenazi Jews' nusaḥ. (via wikipedia).