DISCLAIMER: I love BTTF2 (and it's a really fun story), but it could never have happened as it skips over a very important plot hole in order to propel the story.
I myself ignore this when I watch it, because I really like this film. How. Ever.
You see, Marty and Jennifer leave the timeline. Gone. They vanish in 1985 and reappear in 2015, therefore they were not present in those 30 years to get married and have children as Doc witnessed when he went forward in time alone. Time is a linear experience for the time traveler(s). Doc comes back, grabs Marty and Jennifer, and then alters the timeline, changing the future. The future is what you (ESPECIALLY the person possessing a time machine) make it. Just like Doc's dog Einstein vanishes from reality for the 60 seconds between 1:20 and 1:21 a.m. in the time machine test at Twin Pines Mall. There would be no offspring. No robbery. No chicken threat, no you're-fired-fax.
Marty and Jennifer would have been confronted with the reality that everyone they know and love would have been looking for them for 30 years and would likely have presumed them to be dead. I imagine a lot of their loved ones thought Marty kidnapped Jennifer in a murder-suicide after not passing the audition for the school dance.
So there is a Back to the Future Day, but it would have been very, very boring.
Happy Back to the Future Day, everybody!