According to The Washington Post, President Trump likely knew in January that national security advisor Michael Flynn had lied to the FBI about not discussing sanctions with Sergey Kislyak:
President Trump’s personal lawyer said Sunday that the president knew in late January that then-national security adviser Michael Flynn had probably given FBI agents the same inaccurate account he provided to Vice President Pence about a call with the Russian ambassador.
Trump lawyer John Dowd said the information was passed to Trump by White House counsel Donald McGahn, who had been warned about Flynn’s statement to the vice president by a senior Justice Department official.
Today, CNN says McGahn did, in fact, tell Trump in January he thought Flynn had misled authorities and should be fired:
White House counsel Donald McGahn told Trump that based on his conversation with then-acting Attorney General Sally Yates, he believed Flynn had not told the truth in his interview with the FBI or to Pence, the source said. McGahn did not tell the President that Flynn had violated the law in his FBI interview or was under criminal investigation, the source said.
On May 8th, Yates appeared before the Senate Judiciary Subcommittee on Crime and Terrorism and testified she had warned the Trump White House on January 26, 2017 that she believed Flynn was compromised..."because the Russians knew the true contents of Flynn's conversations about US sanctions with Russia's then-Ambassador Sergey Kislyak."
CNN:
At the time McGahn approached Trump, the White House counsel had no information beyond what Yates told him...
A week later, McGahn was provided a transcript of what Flynn and Kislyak discussed and the conclusion was that it was inconsistent with what Pence said publicly he had been told by the national security adviser.
Despite McGahn's recommendation that Trump fire Flynn, the retired lieutenant general was kept on. Flynn was forced out in mid-February after news outlets reported about Yates' warning to McGahn.
At the time, President Trump said he fired Flynn for lying to Vice President Mike Pence. He made no mention of Flynn's lying to federal authorities.
On Saturday, the day after news broke that Flynn had pleaded guilty to lying to the FBI, Trump tweeted the following:
I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!
— Donald J. Trump (@realDonaldTrump) December 2, 2017
If Trump knew Flynn lied to the FBI when he asked FBI Director James Comey to let the Flynn matter go and then ultimately fired Comey for continuing his investigation, the tweet is an admission of obstruction of justice.
Once this became clear, Dowd came forward and said he - not President Trump - authored the tweet and dictated it to the social media director who posted it to Trump's account.
At the same time, Dowd is arguing a president cannot be charged with obstruction of justice.
WH lawyer told Trump that Flynn misled FBI and Pence (CNN)
Trump lawyer says president knew Flynn had given FBI the same account he gave to vice president (WaPo)