Breakdown of houmonbi ni tachiaimasu.

Questions & Answers about houmonbi ni tachiaimasu.
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。
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