No Need to Have One
So putting the heading and subheading above together means that you don't need to have a cow (, man,) to celebrate Watch a Cow Day.
Those of you who live on a farm - dairy or beef cattle - or out in the country will have a much easier time with this special day than those living in the city or the 'burbs.
If you live near a cow, you might just have to look out your window to watch one or maybe walk out to the barn. If you live farther away, you'll probably have to drive a bit and watch one out your car window.
I think there's an implication here that you watch the cow most of the day, but that's probably not practical for most people. If you did, someone else would likely have a cow - or at least a bird.
What should you watch for?
Good question, since normally cows don't do a whole lot. It must be pretty boring to be a cow.
You stand, lie down, eat, chew cud, swish your tail to keep the flies away. If you're a dairy cow, you let the farmer take your milk. You make cow pies. Other than that, there's not much from day to day.
Then again, if you're a cow, maybe you consider that an exciting day.