The whole day is valid for circumcision, as it says, "and on the eighth day he shall be circumcised" (Leviticus 12:3). But the eager rise early for the commandments, as it says, "And Abraham rose early in the morning" (Genesis 19:27).
The Eager Rise Early to Perform the Commandments
Yalkut Shimoni on Torah 86:2
כָּל הַיּוֹם כָּשֵׁר לְמִילָה, שֶׁנֶּאֱמַר, "וּבַיּוֹם הַשְּׁמִינִי יִמּוֹל", אֶלָּא שֶׁזְּרִיזִין מַקְדִּימִין לְמִצְווֹת, שֶׁנֶּאֱמַר, וַיַּשְׁכֵּם אַבְרָהָם בַּבֹּקֶר.