Ramban

AND MOSES SAID: THUS SAITH THE ETERNAL: ABOUT MIDNIGHT WILL I GO OUT INTO THE MIDST OF EGYPT. Moses said this to Pharaoh and to his servants, as he mentioned at the end of his words, And all these thy servants shall come down unto me. Now he did not explain on what night this plague will take place, since this Divine communication to Moses and the transmission thereof to Pharaoh happened before the first of Nisan, the first of Nisan. In my opinion, this is to be explained on the basis of that which Ramban wrote in Seder Shemoth (4:21), that at the very first time that Moses came before Pharaoh, he warned him of the last plague which will force him to let the people go. In that case, the Divine communication to Moses and the transmission of it to Pharaoh did indeed take place before the first of Nisan. The verse here, yet one plague more will I bring upon Pharaoh, must then be understood in the sense that G-d told Moses that the time had come for the final plague of which He had told him in the beginning and which Moses had related to Pharaoh. See also my Hebrew commentary, p. 323, for further discussion of this problem. and when he said about midnight, he did not yet know on which night it would be. Thus Moses did not inform them of the specific night on which this plague would take place, but instead he said in kindled anger, “I will see thy face again no more. It is you who will call upon me at about midnight to go out from your country, and your servants will bow down to me.” In the second section He elucidated to Israel, And I will go through the land of Egypt in that night, and in the third section Scripture states, And it came to pass at midnight,, Verse 29. i.e., the night mentioned when they were eating the paschal lamb.