I honestly don't think there needs to be a worldwide day of thankfulness. Thankfulness to me, is a state of mind more than anything. They could declare a worldwide thankful day, but unless I chose to be thankful, I would not be. It'd just participate for the food probably.
Yes being that I am a natural born pessimist, yes I do feel that positive is often overlooked by the negative. It's an easy thing do to, dwell on the negative, instead of what's good about any situation. It's easy to sink oneself into having no hope, or bashing others, when really it's myself I should be holding to account.