日本にいるあいだ
While I'm in Japan

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We use VERB ＋ あいだ（に）to express "while".

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日本にいるあいだ に - While I'm in Japan

日本にいるあいだに、まつりに行きたいです。

• While I'm in Japan, I want to go to a festival.

In this case, we add に after あいだ because the 'go to a festival' part is a singular activity that 'punctuates' the time we're in Japan. It won't be going on the whole time. 
​Compare to the example below:

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日本にいるあいだ - While I am in Japan　

​​日本にいるあいだ、ずっと 和食をたべるつもりです。

• While I'm in Japan, I intend to eat Japanese food the whole time.

We don't use に after あいだ in this case because the 'eat Japanese food' part is an activity that I intend to continue the whole time I'm in Japan. 

日本にいるあいだ - While I was in Japan　
​(The あいだ part of the sentence stays in the non-past. The tense is determined by the final verb.)

​​私たちが日本にいるあいだ、ずっと ふとんでねました。

• While we were in Japan, we slept on futons the whole time.

We don't use に after あいだ in this case because the 'slept on a futon' part was an activity that continued the whole time we were in Japan. 

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Here's a pair of examples that express basically the same thing, one with and one without に. Can you work out the difference?

​​私が日本にいるあいだ、びょうきでした。​

​​私が日本にいるあいだ に、びょうきになりました。​

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