getuyou no kawari ni mokuyou ni kaigi wo surukoto ni natta.

The noun かわり usually connects to another noun with の.

So:

• 月曜のかわりに = instead of Monday
• A の かわりに B = B in place of A

Here, 月曜 is a noun, so の links it to かわり.

This is a very common pattern:

• お茶のかわりにコーヒーを飲む = drink coffee instead of tea
• 私のかわりに彼が行く = he will go instead of me

Without の, it would sound ungrammatical.

Here, かわりに means instead of or in place of.

So:

• 月曜のかわりに木曜に = on Thursday instead of Monday

Be aware that かわりに can have other meanings in other contexts, such as in exchange for or as compensation for, but in this sentence it clearly means a substitution of one day for another.

The に after 木曜 marks the time when the meeting will take place.

• 木曜に = on Thursday

Days of the week often take に when you want to mark the specific time of an event:

• 月曜に行く = go on Monday
• 木曜に会議をする = have the meeting on Thursday

In casual speech, this に is sometimes omitted with time expressions, but here it is completely natural and clear.

They are doing different jobs:

1. 月曜のかわりに

   • に is part of the fixed expression かわりに
   • meaning: instead of Monday

2. 木曜に

   • に marks the time: on Thursday

3. することになった

   • に is part of the grammar pattern ことになる
   • meaning: it has been decided that... / it ended up that...

So even though the same particle appears three times, each one belongs to a different structure.

Both are possible in Japanese, but they mean slightly different things.

• 会議をする = to hold/conduct a meeting
• 会議がある = there is a meeting

This sentence uses 会議をする, which focuses on the action of having or conducting the meeting.

So:

• 木曜に会議をすることになった = it was decided that we will hold the meeting on Thursday

If you said:

• 木曜に会議があることになった

that would mean something more like:

• it was decided that there will be a meeting on Thursday

Both can work, but 会議をする is very common when talking about arranging or scheduling a meeting.

Verb dictionary form + ことになる is a very common grammar pattern.

Here:

• する = do
• すること = the act of doing / doing it
• することになる = it becomes the case that one will do it
• することになった = it has been decided that one will do it / it ended up being decided that one will do it

So the sentence means that the plan or arrangement changed, and now the meeting is to be held on Thursday.

A key nuance: ことになった often sounds like the decision came from circumstances, rules, or a group decision, not just from the speaker alone.

This is one of the most common learner questions.

• ことにした = I/we decided to do it
• ことになった = it was decided / it has been decided

So:

• 木曜に会議をすることにした

  • someone actively made the decision
  • often implies the speaker or decision-makers chose it

• 木曜に会議をすることになった

  • the decision is now the situation
  • often sounds more neutral, less focused on who decided

In this sentence, ことになった fits well because it sounds like a schedule change or organizational decision.

Because なる cannot directly attach to a plain verb like する in this pattern.

The こと turns the verb phrase into a noun-like unit.

• 会議をする = to hold a meeting
• 会議をすること = holding a meeting / the act of holding a meeting

Then that whole action can become the thing that was decided:

• 会議をすることになった = it became decided that we will hold a meeting

So こと is doing a nominalizing job here.

Because the past tense applies to the decision, not to the meeting itself.

• 会議をする = the meeting will be held
• ことになった = it became decided / it was decided

So the sentence means:

• the decision was made already
• the meeting itself will happen later

This is very natural in Japanese.

Compare:

• 日本に行くことになった。
  • It has been decided that I will go to Japan.

The going is future, but the deciding is already complete.

Japanese often omits subjects when they are understood from context.

In this sentence, the important information is:

• Monday was replaced
• Thursday is the new day
• the meeting is what was rescheduled
• it has been decided

The listener can usually infer who is involved from the situation, such as we, the company, the team, etc.

A fuller version could be something like:

• 私たちは月曜のかわりに木曜に会議をすることになった。
• 会社では月曜のかわりに木曜に会議をすることになった。

But Japanese often leaves that out if it is obvious.

Both are correct.

• 月曜 / 木曜 are slightly shorter and more conversational
• 月曜日 / 木曜日 are a bit fuller and sometimes a little more formal or careful

In everyday Japanese, the shorter forms are extremely common:

• 月曜に
• 木曜に

So this sentence sounds very natural.