Commuters and occasional weekend train riders going into Manhattan can pick up some seasonal gifts at the Grand Central Terminal Holiday Fair, which opened on Monday, Nov. 12 and runs through Christmas Eve.
— The information here is from this announcement. and the YouTube video.

Image from a Youtube video posted by organizers
Grand Central Holiday Fair
A total of 40 vendors have booths at the event, with 36 taking over half of Vanderbilt Hall and another four in the GCT Main Concourse. The fair takes up a total of 6,500 square feet in the building and gets one million visits a day, according to the organizers.

Images from Metro-North's Grand Central Terminal website
Key for the map, below
One of the longest-running indoor holiday fairs in New York City (it started in 1993), the six-week fair focuses on American-made and handmade products with locally-sourced materials or what the organizers call “a socially-conscious business model.”
For almost six weeks, they’ll be selling artwork, clothing for all ages, men’s and women’s accessories, children’s clothing and toys, as well as home goods. There will be a range of prices.
Hours
Weekdays: 10 a.m. to 8 p.m.
Saturday: 10 a.m. to 7 p.m.
Sunday: 11 a.m. to 6 p.m.
(Closed Thanksgiving • Christmas Eve hours: 10 a.m. to 6 p.m.)

Image from Metro-North's Grand Central Terminal website
Vendor map for the Grand Central Holiday Fair 2018
Here’s a YouTube video showing the fair being put up and giving you a sense of what it looks like, if you haven’t seen it: