"I had called upon my friend Sherlock Holmes on the second morning after Christmas..."
It's pretty easy to determine that Watson was talking about the 27th of December, but what was the year? All The Canon tells us is that it was after 'The Man with the Twisted Lip' (TWIS), 'A Case of Identity' (IDEN), and 'A Scandal in Bohemia' (SCAN). Taking a look at the chronologists shows that there is a large number that place it in 1889, including me. (A total of 18, to be exact.) Only one places it in 1888, and four put in in 1890.
The one placement that is curious is where the eminent Sherlockian William S. Baring-Gould puts it in his famous Annotated Sherlock Holmes. He says the year is 1887. The reason that he does so is mainly because of his dating of TWIS. His placement of that case causes him to be alone in his dating on a number of cases around that time, but even in the face of total disagreement he holds his ground.
Part of Baring-Gould's reasoning is that he thinks Watson is married to mysterious wife No. 1 at this time. (Something else almost no one agrees with.) However, he does say in annotation no. 11 that there is evidence for 1889, and does the same in BLUE, but seems to disregard this evidence. Two of the biggest problems with 1887 for TWIS is that Holmes actually mentions that year in conversation, and that he (Baring-Gould) ignores something else that happened around that time that should have been mentioned but wasn't. Bradley & Sarjeant point out that Queen Victoria's Golden Jubilee would have been on the 20th & 21st of June 1887. That's just two days after Baring-Gould's date. It seems that the preparation and excitement for such a large celebration would have gotten some mention, and yet it doesn't.
As stated before, there are only a few who disagree with 1889 for BLUE. Yes, there is a problem with the day of the week, but you can insert any reason you want into the argument for why Watson made that mistake. One other large piece of evidence for BLUE being that year is that the case is mentioned in 'The Adventure of the Copper Beeches' (COPP), and that one is (again) almost unanimously agreed upon as occurring in early-to-mid 1890.
When I tackled the story in my chronology column a couple of years back I found no reason to disagree with December 27, 1889. However, a lot of dates might get changed if anyone ever comes up with a reasonable conclusion to the Wives of Watson problem. Until then I cannot see a reason to doubt The Good Doctor's date.
(I didn't intend to tackle this case quite so quickly, but given the date I had to. You understand.)