Just did some experiments with a few friends. It seems Niantic has keyed in the free incubator giveaways to follow PST time for the whole week. What does this mean?
To get an incubator, you must follow to rather disparate criteria:
It must be the first spin of the day in your local timezone
You must have not gotten an incubator in Niantic's PST timezone between 00:00 and 23:59 that specific day.
So what does this mean?
In England, right now, it's 5:17am, December 26. In San Francisco, it is 9:17pm, December 25. I just did my Pokestop spin for December 26, however, there was no incubator - because in SF it is still Dec 25, and I already have Dec 26's incubator. This means I will have to spin again at midnight when it's the 27th here and the 26th in SF to get Dec 26's incubator.
If you spun too early, but it's a new day for you, without being a new day for Niantic - you get another chance!! GET ON IT. Even if you're on the east coast and spin when it's the 26th for you but still the 25th for niantic, you get your dec 25 incubator that you missed!