الْيَوْمَ أَكْمَلْتُ لَكُمْ دِينَكُمْ
Today I have perfected your religion for you.
In situational or present familiarity, the setting itself identifies the referent; «اليوم» points to a particular day in the speech situation. In mental familiarity, speaker and hearer share knowledge of a referent even if it is not physically present.
Both are forms of identifiable reference, but the basis of identification differs: immediate situation versus shared understanding. Not every definite article points to something mentioned earlier in the text.