Full details of the deal are not known, but Christmas said following the Celtics' 92-77 summer league loss to the Los Angeles Clippers that the contract will be partially guaranteed.
"I actually got the call before the game started," Christmas said. "I don't really know all the details now, but I'm going to go over them with the staff. But I'm just happy, man, emotional. I actually got the call literally two minutes before the game started."
The Celtics and Christmas' representatives had been discussing a potential deal all week, with Christmas seeking some sort of guarantee from the club. He said he had received significant offers to continue playing in Europe, where he had spent the majority of the last three seasons, but Christmas maintained throughout the week that his ultimate goal was to make it to the NBA this coming season.
"That was my whole mindset," Christmas said, "to come in here and try to get (the Celtics) to like me and bring me in, and it happened."
Christmas also later tweeted: "Wanna thank the @celtics for giving me this opportunity to make my dream come true ....#wordscantexplain".
Christmas, 25, went undrafted in 2009 after a four-year career at Temple.
In Summer League play, Christmas averaged 14.2 points, 4.2 rebounds, 2.2 assists, and shot 47.9 percent from the field overall and 45 percent from the 3-point line.