Erik and Lyle Menendez will NOT have their day in court this month … because their upcoming resentencing hearing has been postponed, and it’s all because of California Governor Gavin Newsom.

The Menendez Brothers were set to enter a courtroom March 20 and 21 for the first time since their 1990s murder trials … but now their resentencing hearing is delayed because Newsom ordered an investigation into whether they are fit to return to public life.

Resentencing is now irrelevant for Erik and Lyle, with Newsom short-circuiting the whole process by directing the state’s Board of Parole Hearings to conduct a “risk assessment investigation” on the brothers.

Lyle reacted to the news on social media, posting a message on Facebook from his San Diego prison … telling his family and supporters to “Hang tight, be patient, and keep the faith.”

TMZ broke the story … Newsom told us experts will analyze “what led each brother to commit their offense, and if they have done the transformational work needed — and gained the skills — to avoid repeating their mistakes.”

The major, unexpected development came one week after Erik and Lyle appeared on TMZ’s “2 Angry Men” podcast, where they told Harvey and Mark Geragos (their attorney) about their lives behind bars.

Newsom told us the parole board will get the results of the risk assessment in a few months and hold a parole hearing … which is one path to freedom. The governor also has the power to grant clemency to the Menendez bros.