"And they journeyed from Elim, and all the congregation of the children of Israel came [to the wilderness of Sin]" (Exodus 16:1). Why is the word "day" stated? It teaches that on that very day it happened that a Shabbat fell, for the Shabbat is set in order and comes around from the day the Holy One, blessed be He, created it until He gave it to them on the twenty-second of Iyyar.
The Day Israel Reached the Wilderness of Sin Was a Shabbat
Yalkut Shimoni on Torah 257:6
(שמות טז א) וַיִּסְעוּ מֵאֵילִם וַיָּבֹאוּ כָּל עֲדַת בְּנֵי יִשְׂרָאֵל. לָמָּה נֶאֱמַר יוֹם, מַגִּיד שֶׁבְּאוֹתוֹ הַיּוֹם אֵרַע שַׁבָּת לִהְיוֹת, שֶׁהִיא סְדוּרָה וּבָאָה מִיּוֹם שֶׁבָּרָא הַקָּדוֹשׁ בָּרוּךְ הוּא עַד שֶׁנְּתָנָהּ לָהֶן בְּעֶשְׂרִים וּשְׁנַיִם בְּאִיָּיר.