Jamieson, Fausset and Brown on Deuteronomy 16:2

2. Thou shalt therefore sacrifice the passover--not the paschal lamb, which was strictly and properly the passover. The whole solemnity is here meant, as is evident from the mention of the additional victims that required to be offered on the subsequent days of the feast (Nu 28:18, 19; 2Ch 35:8, 9), and from the allusion to the continued use of unleavened bread for seven days, whereas the passover itself was to be eaten at once. The words before us are equivalent to "thou shalt observe the feast of the passover."