This was acutally the day of preparation of the Passover. This is drawn out more clearly in John.
14 Now it was the Preparation Day of the Passover, and about the sixth hour. And he said to the Jews, “Behold your King!” (19:14, cf. John 19:31,42--nkjv)
Possibly connected with Exodus 12 Which depicts the first day of Unleavened Bread as a day in which no work was to be done; only preparation for the Feast of Unleavened Bread was allowed:
14 'Now this day will be a memorial to you, and you shall celebrate it
as a feast to the LORD; throughout your generations you are to
celebrate it as a permanent ordinance. 15 'Seven days you shall eat
unleavened bread, but on the first day you shall remove leaven from
your houses; for whoever eats anything leavened from the first day
until the seventh day, that person shall be cut off from Israel.
16 'On the first day you shall have a holy assembly, and another holy
assembly on the seventh day; no work at all shall be done on them,
except what must be eaten by every person, that alone may be prepared
by you. 17 'You shall also observe the Feast of Unleavened Bread, for
on this very day I brought your hosts out of the land of Egypt;
therefore you shall observe this day throughout your generations as a
permanent ordinance. 18 In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. (nkjv).
See also, Luke 22:7-9 where the word "prepare" is used twice in reference to this day on which the lamb was killed.
7 Then came the first day of Unleavened Bread on which the Passover
lamb had to be sacrificed. 8 And Jesus sent Peter and John, saying,
"Go and prepare the Passover for us, so that we may eat it." 9 They
said to Him, "Where do You want us to prepare it?"
See also notes in Bullinger's Companion Bible for Mark 15:42.