Visitors ask...Jubilate Deo Sunday or Jubilate Deo Mass

I presume this refers to "Jubilate Sunday." As the inestimable New Advent Catholic Encyclopedia explains,

The third Sunday after Easter, being so named from the first word of the Introit at Mass — "Jubilate Deo omnis terra" (Psalm 65).

The Introit for this Mass is:

Jubilate Deo, omnis terra, alleluia: psalmum dicite nomini ejus, alleluia: date gloriam laudi ejus, alleluia, alleluia, alleluia. -- Dicite Deo, quam terribilia sunt opera tua, Domine. In multitudine virtutis tuae mentientur tibi inimici tui. V.: Gloria Patri . . . -- Jubilate Deo, omnis terra . . .