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.

• 今（いま）は仕事（しごと）中（ちゅう）なんだ。
  I am in the middle of working right now.
• 締（し）め切（き）りは今月（こんげつ）中（ちゅう）なんです。
  The deadline is sometime this month.

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.

• 彼（かれ）は休憩（きゅうけい）中（ちゅう）に本（ほん）を読（よ）む。
  He reads books during his break. (At some point during the act of taking a break)
• 私（わたし）は勉強（べんきょう）中（ちゅう）に話（はな）されるのが嫌（きら）いだ。
  I don't like being talked to while studying. (At some point during the act of study)
• 今日（きょう）中（じゅう）にレポートを提出（ていしゅつ）してください。
  Please submit your papers by today. (Within today as the limit)
• 今年（ことし）中（じゅう）にイタリアに行（い）きたい。
  I want to go to Italy this year. (Within this year as the limit)

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.

• 食事（しょくじ）中（ちゅう）に電話（でんわ）がかかってくると、イラっとする。
  It ticks me off when I get a phone call while I'm eating.

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.

• ゴミが家（いえ）中（じゅう）に広（ひろ）がっていて、歩（ある）くスペースがない。
  There is so much garbage throughout my house, that there is no room to walk.