To Cheer up a home Sick Sunset , Pinkie Pie invents “Pony Day” a yearly event where every one will party while on all 4s surrounded by cheep cardboard recreations of What Pinkie imagines Ponyville to look like and dressed up to look like ponies,  
 
Pony costumes can be pony onesies, or minimal coverage such as shirts with out pants etc , most will show up in swim suits with clip on tails and ears . clip on horns and wings for some.  
 
there will be games based on not using your hands and playing whatever other activity Sunset wants to do.  
 
Its a day to cut loose and let humans do things ponies do to make a pony who is now a human feel more like a pony again.