I wanted to share my experience for posterity since previous threads provided very different timelines from this year. To those still waiting, I unfortunately have no idea if the foundation sends out all of the letters at one time or if it varies by department. I hope you hear back soon!
I received an interview request from one department mid-December 2018 and the second in early January. Both interviews ended up being mid-January. I heard back from the department/one of my interviewers in early February offering me the internship. The official award letter from the foundation did not arrive until March 4th. I don't know if the offer from the department (before foundation award) is standard or not.
Anyone else not able to create an application for the Masters program at Williams? On their application page it only gives the option to put in a preexisting PIN and password, but I never made an application. Tech support told me to click "create" and application on the far right, but I don't see that option anywhere on the screen.