Most of the Usual Suspects highlight these two observances. We would prefer that Common Courtesy Day to be observed each and every day, but if everyone observes it on Thursday, that might be a nice start.
And it's hard to argue with the idea of setting aside a day to offer support for the single parents we might know. Of course, single parents need your support far more often than just on Single Parent's Day.
Thursday is also National French Bread Day and a UN observance, the International Day for the Elimination of Racial Discrimination (the theme of this year's observance being "racism and sport").
There is some confusion as to whether March 21 is Brain Injury Awareness Day or whether it was supposed to be observed on March 13. Either way, March is Brain Injury Awareness Month, so Thursday is as good a day as any to note the observance.