houmonbi ni tatiaimasu.

It is read:

ほうもんび に たちあいます。

A couple of useful notes:

• 訪問日 = ほうもんび
• 立ち会います = たちあいます
• In normal Japanese writing, it would usually appear without spaces: 訪問日に立ち会います。

訪問日 is a compound noun:

• 訪問 = visit
• 日 = day / date

So 訪問日 means the day of the visit or the visit date.

In business or administrative Japanese, this kind of compound noun is very common. It sounds more formal and compact than a longer phrase like 訪問する日.

Here, に marks the time when something happens.

So:

• 訪問日に = on the visit day / on the date of the visit

This is a very common use of に with specific points in time.

Compare:

• 月曜日に行きます。 = I will go on Monday
• 3時に始まります。 = It starts at 3 o’clock
• 訪問日に立ち会います。 = I will be present on the visit day

立ち会います is the polite -ます form of the verb 立ち会う.

Dictionary form:

• 立ち会う

This verb means something like:

• to be present
• to attend
• to witness
• to be on hand for something

It often has a slightly formal or practical feel. It is commonly used in situations like:

• inspections
• meetings
• procedures
• site visits
• official events

So it is not just to go with someone in a casual sense. It often means to be present for an event or process.

Grammatically, 立ち会います is in the non-past form.

In Japanese, the non-past can mean:

• present
• future
• habitual action

In this sentence, it will usually be understood as future because 訪問日 gives a specific time point.

So in context, it most likely means:

• will be present on the visit day

The subject is omitted.

This is very normal in Japanese. If the subject is already clear from context, Japanese often leaves it out.

Depending on the situation, this could mean:

• I will be present
• we will be present
• he/she will be present

If you want to state the subject explicitly, you could say:

• 私は訪問日に立ち会います。 = I will be present on the visit day.
• 担当者が訪問日に立ち会います。 = The person in charge will be present on the visit day.

Yes, it can be natural if the context already makes it clear.

However, 立ち会う often refers to being present at some event, process, or activity, so in isolation the sentence can feel a little context-dependent.

For example, in a real conversation, the omitted thing might already be obvious:

• a house inspection
• a delivery
• a site visit
• a procedure
• a meeting

So 訪問日に立ち会います。 is understandable as a short practical statement, especially in workplace or scheduling contexts.

If you wanted to be more explicit, you might say something like:

• 訪問日に現場確認に立ち会います。
  = I will be present for the on-site inspection on the visit day.

Both are possible, but they feel different.

• 訪問日 is more compact and formal
• 訪問する日 is more explanatory and neutral

So:

• 訪問日に立ち会います。 sounds concise, like business Japanese
• 訪問する日に立ち会います。 sounds a little more descriptive

In schedules, notices, forms, and workplace communication, Japanese often prefers compact noun compounds like 訪問日.

Usually, に is the natural choice here because it marks the time of the action.

• 訪問日に立ち会います。 = I will be present on the visit day

If you use は, the nuance changes:

• 訪問日は立ち会います。

This sounds more like:

• As for the visit day, I will be present
• or a contrast, like I will be present on the visit day, though maybe not on other days

So に is the normal time marker, while は gives topic or contrast.

Those spaces are usually just for learners.

Standard Japanese is normally written without spaces:

• 訪問日に立ち会います。

Teaching materials sometimes insert spaces to make it easier to see:

• where one word ends
• where a particle is
• what the verb is

So the spaces are not wrong as a learning aid, but they are not how Japanese is usually written in real life.