Take a Walk in the Park Day is celebrated on March 30th of each year. The staff at National Whatever Day were unable to discover the origin of Take a Walk in the Park Day.
Walk in the park could be taken in the literal sense in which one take a recreational walk in the park. A little bit of exercise never hurt anyone, right?
Walk in the park could also be used as something easy or pleasant, especially by comparison to something. (i.e. High school was difficult, but it was a walk in the park compared to college engineering classes.