Moses and Aaron then went to Pharaoh and said, 'This is what YHVH, God of the Hebrews, declares: 'Let My people leave, so they can sacrifice to Me in the desert.' '
Ve'achar ba'u Moshe ve'Aharon vayomeru el-Par'oh koh-amar Adonay Elohey Yisra'el shalach et ami veyachogu li bamidbar.
(Ibn Ezra; Radak, Sherashim; Ralbag). Or, 'celebrate a festival to Me' (Targum Yonathan). In the Talmud both opinions are cited (Chagigah 10b).