Donald Trump Continues Thank-You Tour, Will Formally Announce James Mattis as Secretary of Defense Pick

President-elect expected to join retired Gen. James Mattis at event.

After a busy day of meetings at Trump Tower in New York, the president-elect is scheduled to fly out later today for the 7 p.m. ET rally.

He teased the announcement of Mattis to attendees, saying, "We are going to appoint 'Mad Dog' Mattis as our secretary of defense. But we're not announcing it until Monday, so don't tell anybody. Mad Dog, he's great. He is great," Trump said, using one of Mattis' nicknames.

Trump will travel later this week to Iowa and Michigan, two other battleground states that were vital to his win.

ABC News' Katherine Faulders contributed to this report.