The historical answer is, on the day when Israel stood at Mount Sinai, for that is when the commandments were given, and that’s why we celebrate Shavu’ot on 6 Sivan.
However, that answer does not seem to work when we read this week’s portion. In this context, “this day” cannot mean 6 Sivan, so how do we handle the problem? We cannot abolish or deny the Shavu’ot date, so what do we do?
Rabbinic commentary makes a wise choice when it says, “Though the commandments were indeed given on a set day, we must discover them anew every day of our lives. When we observe them it must not be out of ancient habit alone, but because we find them fresh and relevant every single day we live.”
An example is the requirement of justice: it has been around for a very long time, but every day we have to accept it anew, and employ and apply it in our own set of circumstances.
At all times justice calls for us to be fair to other people. There are times when the need of the hour is to be just and fair to oneself.
Another example is the requirement of peace. The concept is eternal but at any given time it needs to suit the circumstances.
There are moments for a peace-offering; there are other moments when peace-offerings are sheer appeasement. History laughs at Neville Chamberlain for coming back from Hitler (yimmach sh’mo) boasting that he had achieved “peace in our time”.
Note that when the Bible speaks of justice and peace it uses the same verb, to pursue: “Justice, justice shall you pursue” (Deut. 16:20), “Seek peace and pursue it” (Psalm 34:15). The pursuit does not always take the same form.