This does not refer to sundown. Rather, the light of the sun darkened during the middle of the day. Use a term to describe the sun becoming dark rather than the sun going down.
"the curtain inside the temple." This was the curtain that separated the most holy place from the rest of the temple.
# the curtain of the temple was split down the middle
"the curtain of the temple was torn from top to bottom into two pieces." This can be stated in active form. AT: "God tore the temple curtain into two pieces from the top to the bottom" (See: [[rc://en/ta/man/translate/figs-activepassive]])