Why was the expression "Today" mentioned in the noble verse: ﴿This day are (all) good things made lawful for you. The food of those who were given the Scripture is lawful for you﴾, even though Allah Almighty did not forbid the slaughtered animals of the People of the Book neither before nor after the revelation of this Surah?

The word "today" in the verse refers to the day previously mentioned in His (the Almighty's) saying: "This day, those who disbelieve have despaired of [defeating] your religion; so fear them not, but fear Me. This day, I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion." (Al-Ma'idah/ 3). The purpose of its repetition is to completely announce the permissibility of good things (Tayyibat), and that is part of the perfection of the blessing in worldly matters, not that it was forbidden before that.