"This would be so easy for Mexico to do, but they just take our money and 'talk.' Besides, we lose so much money with them, especially when you add in drug trafficking etc.), that the Border closing would be a good thing!"
The big picture: While Trump has made a similar threat on Twitter before, he hasn't attached a timeline in the past. It's not clear exactly how he'd follow through on his threat, though the Washington Post reported last year that he planned to deny asylum to migrants at the border using proposed emergency powers similar to the 2017 travel ban.