As I don't have a dog here in Japan, the main attraction for me was that there were cute dogs everywhere! I will never stop being surprised by how many Japanese people dress their dogs up in all sorts of costumes and quite often push their pups around in prams. Sometimes you'll see someone pushing around a cart filled with about 5 dogs in which is quite a sight to behold.
If you're a dog lover I'd definitely recommend checking out one of these festivals. The dog owners are very happy for you to come over and take photos and pet their dogs. They are like proud parents. A kind lady even let my boyfriend hold her Shiba Inu puppy and I don't think he's ever been so happy in his whole life.
(There is no date set for next years carnival but keep an eye on the carnival's website next April)
Osaka Dog Café
Tokyo Dog Café