It was Christ’s own sacrifice of Himself - but by His words ‘do this in memory of Me’ He commands us to keep offering the same sacrifice of Himself to the Father. And this is what we do in the Eucharist.
Just as God, through Moses, commanded the Israelites to celebrate Passover again each and every year.
Interestingly, if you know anything of biblical languages, an unusual word is used in biblical accounts of the OT celebration of Passover. The actual Greek word escapes me, (I think it’s ‘amenesis’?) but it signifies that these yearly Passovers were not merely remembrances of a one-off long-ago event, but that in some mystical way the Jews were present again at the same original Passover each year in their yearly celebrations.
That is also how we view the new Passover, the Eucharist. It is one and the same sacrifice - the sacrifice of Calvary - at which we are present again and again.