Donald Trump Jr., the eldest son of the former president, will testify Monday in his father’s civil fraud trial, making a return appearance in a proceeding that has featured a parade of family members on the witness stand.

He is being called by defense lawyers as they begin their arguments in the trial, which began five weeks ago and could last until mid-December.

The case against Donald J. Trump stems from a lawsuit brought by New York’s attorney general, Letitia James, which accuses Mr. Trump and other defendants, including his companies and his sons Donald Jr. and Eric, of fraudulently inflating the value of assets to obtain favorable loans and insurance deals. The state rested its case last week.

In his first appearance, on Nov. 1, Donald Trump Jr. testified after being called by prosecutors that he had no direct involvement in annual financial statements that his family’s business gave banks and insurers, despite language in the statements suggesting that he was partially responsible for them.

Eric Trump and Donald J. Trump have also testified in the trial, as has Ivanka Trump, who is not a defendant. The family members’ testimony has drawn the spotlight to a proceeding that has primarily been a long slog through the minutiae of Trump Organization finances.

Here’s what else you need to know:

  • Before the trial, the judge, Arthur F. Engoron, found that the financial statements were fraudulent, granting Ms. James a major victory. The trial will decide punishments that Mr. Trump may face. The attorney general has asked that he be fined $250 million and be permanently barred from doing business in New York.

  • Right-wing allies are going after the judge on Mr. Trump’s behalf, through official channels and online. Representative Elise Stefanik, a member of the House Republican leadership, filed an ethics complaint Friday against Justice Engoron.

  • When Donald J. Trump took the stand, he delivered a bombastic performance in which he assailed Ms. James, a Democrat, as a “political hack” and denounced the trial as “very unfair.” He denied involvement in fraud but acknowledged a role in creating the financial statements.

  • Justice Engoron has barred Mr. Trump from making comments about his legal staff. The former president has twice violated that order, incurring fines that amount to $15,000.