a common Jewish observance/practice. gills talks about it and there's a short explanation from
http://www.hebrew-streams.org/works/ntstudies/yeshua-sang.html
Yeshua Sang Hymns
by Paul Sumner
(1) Matthew 26:30
After singing a hymn,
they [Yeshua and his disciples] went out to the Mount of Olives.
This event followed the Last Passover at which Yeshua told his disciples to remember him and his sacrifice with bread and wine.
In ancient Passover services, as recorded in the Mishnah, the meal ended with singing the so-called "Second Hallel" (Psalms 115–118). Then came the Fourth Cup, following by the "Blessing of the Song" (a kind of reprise of previous hymns sung during the Seder) (Mishnah, Pesachim).
The phrase translated "singing a hymn" in Matthew 26:30 is one word in Greek: humneo. It means to hymn, sing praise, celebrate or worship with hymns. Thus, v. 30 says, "after they hymned" or "after hymning" (Grk, humnesantes) they left the upper room where the meal was enjoyed.
In Modern Hebrew NTs, the phrase in v. 30 is rendered two ways: "after concluding the Hallel" (gamar haHallel; Delitzsch, Salkinson) or "after singing the Hallel" (shar haHallel; Israel Bible Society, 1991).
Yeshua not only participated in the Passover—as its presiding host—he joined all Israel that night in singing hymns of praise to God. He and they thanked and worshiped the One who rescued his people from Egyptian bondage some fourteen centuries before.
A Messianic Hymn
Yeshua's last hymn was the Second Hallel. Its concluding psalm, 118, reverberates with messianic meanings.
Part of Psalm 118 tells how a decision by Jerusalem's builders to reject a certain "stone" was overruled by God. What they rejected, God laid down as the Rosh Pinnah, chief cornerstone, of his Temple. His sovereign action was "marvelous" in the eyes of the faithful; it was a "day which the LORD [had] made" — and thus a day to be rejoiced in (vv. 22-24).