中(ちゅう)During, While, Throughout, In the process of

As a suffix, 中(ちゅう) is frequently used to highlight a period of time (or space). By attaching the case marking particle に, the sentence can be modified, to show that (B) is happening/existing throughout the timeframe/space of (A). This is often translated as 'throughout (A), (B)', or 'during (A), (B)'. First, let's take a look at 中(ちゅう) being used to show events that are in process.

Now, to show that a (B) event is happening within (A), simply add the case marking particle に to show a 'temporal location'. This will be done by attaching に to the end of any noun that you would like to indicate as the being the 'space' within which another thing exists/occurs.

In these examples, we can see that 中(ちゅう)に is sometimes read/said as 中(じゅう)に. The basic rule for when to use ちゅう or when to use じゅう is as follows:

中(ちゅう)に - Expresses a very specific (usually small) amount of time/space, or something that is not always the same (an amount of time/space that can change, depending on the person/situation). This 'thing' that ちゅう highlights is usually a 'process', and therefore means that (A) is 'in the process' of happening.

Examples - A meeting, class, visit to somewhere.

中(じゅう)に - Expresses a predetermined amount of time/space that will always remain the same, regardless of the person/situation. Therefore, じゅう does not highlight a 'process', but focuses on the 'length' or 'space' itself. This tends to mean 'all throughout', as it focuses on the full amount of time/space.

Examples - A day, year, house, town or country.

Caution

There is also a difference between 音読(おんよ)み (chinese readings of words), and 訓読(くんよ)み (japanese readings of words).

じゅう is used more frequently with 訓読(くんよ)み - (今年中(ことしじゅう), 夏休(なつやす)み中(じゅう), 今晩中(こんばんじゅう)).

ちゅう is used more frequently with 音読(おんよ)み - (水中(すいちゅう), 会議中(かいぎちゅう), 試合中(しあいちゅう)).