That would be tomorrow, as of press time.
Switch Online is continuing to offer game trials, and the next one may sway you toward a game in Nintendo's New Years sale.
North American Switch Online subscribers (Expansion Pass NOT required) will be able to play Captain Toad: Treasure Tracker for free this weekend. Unlike most game trials which provided a few day's notice, the trial period for Captain Toad will begin tomorrow at 1 p.m. ET and run until 2:59 a.m. on January 21.
Another break from tradition may come with a corresponding sale: Nintendo first party titles that have been offered in the past as NSO trials have gone on sale at the same time and remained discounted for up to a week after the trial ends. Captain Toad is already a part of Nintendo's New Year's sale which ends at 2:59 a.m. (ET) on Monday. Nintendo's notification has not mentioned if the sale will be extended.