This story is a sequel to Equestrian Vacation

Harry's time in Equestria has given him a lot. New friends, new magic, and new problems were guaranteed. But now Harry must deal with his inevitable return to Earth. Harry knows Voldemort will try to return to power, and this time he's ready to fight back. He must uncover the secret to the Dark Lord's immortality, and find a way to stop him before it's too late.

Harry Potter and Twilight Sparkle have been best friends and penpals for a long time. She was his first friend, and they became the closest of companions after Twilight found a way to summon Harry to Ponyville.
When Twilight perfects her spell to bring her long time penpal, Harry Potter, to Equestria, she plans a relaxing weekend vacation away from Hogwarts before the Yule Ball. However, fate has other plans as the two are thrust into an adventure of friendship and magic. Now Twilight is more involved with her friend's (and the wizarding world's) affairs than ever, and the bonds of their friendship will be tested.