When did the New Testament begin?

Hebrews 9:1 speaks of a "first covenant", and
Hebrews 9:15 speaks of a "new covenant".

For where a testament is,
there must also of necessity be the death of the testator.
For a testament is of force after men are dead:
otherwise it is of no strength at all while the testator liveth.
Hebrews 9:16-17
This means that the New Testament was not in effect until Jesus died. Jesus died in the Gospels in Matthew 27:50, Mark 15:37, Luke 23:46, and John 19:30.

Here is a sketch to illustrate this point: