Sure, it's as silly as any of the other artificial holidays, but it's my kind of silly! I love learning about the everyday life of days long ago, I love the "fish out of water" concept of someone doing their best to survive in a foreign time period, and when you open your mind to perspectives of thought not of this time period, you learn a lot about yourself. TV shows like Highlander and Doctor Who can hold my attention no matter how bad the writing or character development. The film Somewhere In Time is as sappy as a maple tree in March, but I can watch it over and over and over (and tear up at the end every time). I will read the world's worst writing if the research on the time period is decent and the protagonist is gaining something from their experience.
I don't know who started Pretend To Be A Time Traveler Day, but I plan to celebrate in my own small way. When I'm out and about this afternoon, I'll be doing my best to phrase my speech in the patterns of a late 19th century Londoner, and I'll be wearing a scarf in honor of that very great time traveler, The Doctor.