God made many covenants in the Old Testament, but I assume you
are speaking about the "New Covenant" vs the "Old Covenant" which
God gave Israel through Moses at Mount Sinai. The "New Covenant" or
second was mentioned in the Old Testament several times as a
covenant which was yet future. The "New Covenant" started in the
New Testament when Jesus Christ died on the cross of Calvary, was
buried, and was resurrected the third day.
The 'Old Covenant" was by Law and the "New Covenant" was by
grace.