"And God called the light Day" (Genesis 1:5): He called that which goes on giving light "Day." If so, then "and to the darkness He called Night" means He called that which goes on growing dark "Night." But we hold that until the stars come out it is not yet night. Rather, the Merciful One called the light into being and charged it with the duty of the daytime; the Merciful One called the darkness into being and charged it with the duty of the nighttime.
When Daylight Begins and When Night Truly Falls
Yalkut Shimoni on Torah 5:3
וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם, לַמֵּאִיר וּבָא קָרָא יוֹם. אֶלָּא מֵעַתָּה וְלַחֹשֶׁךְ קָרָא לָיְלָה לַמַּחְשִׁיךְ וּבָא קָרָא לָיְלָה הָא קַיְמָא לָן דְּעַד יְצִיאַת הַכּוֹכָבִים לָאו לַיְלָה הוּא אֶלָּא קָרְיָא רַחֲמָנָא לִנְהוֹרָא וּפַקְדֵיהּ אֲמִצְוְתָא דִּימָמָא קָרְיָא רַחֲמָנָא לַחֲשׁוֹכָא וּפַקְדֵיהּ אֲמִצְוְתָא דְּלֵילְיָא.