Or HaChaim on Exodus 5:6/אור החיים על שמות ה׳:ו׳

1 ויצו פרעה ביום ההוא, On that day Pharaoh issued orders, etc. This directive was to be effective only on that particular day. He hoped that by forcing the people to concentrate on their immediate and overwhelming problem they would forget about their fear of what might happen if they failed to offer sacrifices to their G'd in the desert. Any intelligent person understands that an immediate and pressing problem is apt to push other problems that are not so immediate into the background. Pharaoh hoped that a single day of the new decree would suffice to accomplish this objective.

2On the other hand, the Torah could have indicated with the words ביום ההוא that there was only a single day left on which the Israelites performed slave labour, seeing that on the morrow Moses and Aaron would turn the life-giving river Nile into blood. According to our tradition no more slave labour was performed from the day the plague of blood occurred. Not only that, but the Egyptians would pay the Israelites good money to obtain some drinking water from them seeing that water did not turn into blood once the Israelites held it in their hands (compare Shemot Rabbah 9,10). The expression כתמול שלשום, poses no problem as the Israelites did not complete their quota on the day prior to Moses' interview because it was a public holiday.