It is great to be able to go to the Legal Affairs Bureau alone.

It will be a school holiday next week on a festival day.
It is for spirit this time until time to be able to meet.
聼起來自然嗎?

It is great to be able to go to the Legal Affairs Bureau alone (this means normally you can't go alone, now, finally you can OR you are excited to go alone-but better to leave out "to be able"

It is great to go to the Indian Affairs Bureau alone. (can always go alone but just appreciate going by oneself a lot)

but first sentence you wrote is correct and sounds good.

Next two sentences sound less natural.

Next week, there will be a school holiday on a festival day. (There will NOT I will)

Last sentence not understandable. Just have to guess.

We remember our ancestors' spirits on Festival Day, until we can be with them again.