Sometimes we formulate English for ourselves to use, not minding whether the dictionary recognizes them or not. As long as someone begin to use it and it sounds nice, everyone will start using it without making any further enquires.

The term "Next Tommorow" is totally wrong. The reason why we have "Next week", "Next month" and so on, is because we have "This week", "This month" etc. But we do not have "This Tomorrow" so there is no way we could have "Next Tommorow".

The funniest part of it all is that the correct term is not as popular as the wrong term.

Alternatively, it is referred to as "The day after tomorrow", but that's not the real term. The real term is "Overmorrow".

"Day after tomorrow" is the phrase while the word is "Overmorrow" and not "Next tommorow".

Hope this works helpful? If yes, share please.