Rashi on Exodus 16:1/רש"י על שמות ט״ז:א׳

1בחמשה עשר יום ON THE FIFTEENTH DAY — The day of this encampment is specially mentioned because on that day there came to an end the cake (provisions) they had brought with them from Egypt and they now needed the Manna, thus informing us that of the remains of the dough they had prepared in Egypt (cf. Rashi on Exodus 12:33) they ate sixty-one meals. For the Manna fell for them on the sixteenth day of Iyar, which was the first day of the week, just as is stated in the Treatise Shabbat 87b.