I've perceived an uptick in the use of "Turkey Day" to refer to Thanksgiving, and I ran a basic sanity Check against Google Ngrams. It seems to be on the rise since about 1970, but I also noticed a bump in usage in the late 1910s.
I'm not good at interpreting Ngram data - is that bump meaningful at the given frequency and smoothing? If so, why was the phrase temporarily popular? (Was it even referring to Thanksgiving)?
The peak in the Ngram graph of 'turkey day' frequency of occurrence between approximately 1912 and 1927 may be explained by there being at that time two "turkey days". These were the days before Thanksgiving and before Christmas when dressed turkeys were brought to market. This use is distinct from the use with reference to Thanksgiving Day:
(From Laws Relating to Fur-bearing Animals, 1916.)
(From Butchers' Advocate: Dressed Poultry and the Food Merchant, Volume 64, 1917.)
(From Illinois Central Magazine, Volume 10, Issues 7-12, 1922.)
As can be seen from the clippings, these commercial 'Turkey Days' were a celebration in their own right (much like the contemporary 'Black Friday') and, in addition, as a commercial event, generated much publicity in the periodicals of the time. Those periodicals, in turn, were collected as part of the Google Books corpus, and so explain the prominence of the mentions in the Ngram graph you generated.
By my conservative estimate, 17 of the total 28 hits between Jan. 1, 1917 and Dec. 31, 1922 reflect use of the term 'turkey day' with reference to one or another of the commercial 'turkey days'; 6 of the other hits have no relation to Thanksgiving Day itself. So only 5 of the 28 hits might have reference to Thanksgiving Day. In some cases I was unable to determine what day or day type (commerical sales day or holiday) was referenced.
The nickname "Turkey Day" could be used fondly or derisively, depending on the situation. The bumps in usage can be traced to changes (either positive or negative) in the nation's psyche.
The bump towards the end of the 1910's that you mention could be attributed to America's entering World War One. Another rise, in the 1940's, would then have to do with World War Two, when patriotic feelings ran high. My Ngram graph shows the latest rise beginning in the very end of the 1970's (this might have something to do with the Iran that bruised us, and the subsequent steady rise up until the Clinton era could be ascribed to the Reagan and Bush, Sr. years, when the patriots used the nickname fondly while their opponents resorted to sarcasm). There are ups and downs after that, but it would seem that the bump occurs each time something momentous comes along that in one way or another concerns all Americans at once.
