The month after next he'll fly to Tokyo.

再下个月他将飞往东京。

You can say, " the week after next, " you can say, " the month after next."

你可以说“The week after next” ，你可以说“The month after next” 。

If I were to promise you a holiday on the thirtieth of the month after next, do you think you would forget the date?

“如果我答应你下下个月的30号放假，你会忘记日期吗？”