Learn Hardcore Japanese: asita ha itiniti ie wo rusu ni simasu. - I will be away from home all day tomorrow.

The pattern is:
X を留守にする = “to make X be in a state of absence / unattended”
Literally: 家を留守にします ≈ “I will make the house be in a state of no one home.”

So を is used because 家 is what you are putting into the state of 留守 (absence), not the place where an action happens (で) or a destination/goal (に).

If you said 家にいます, then 家 is the place where you are, so you use に. But with 留守にする, the fixed pattern is X を留守にする.

What does 留守にする literally mean, and how is it different from just saying “to go out”?

留守 (るす) by itself is a noun meaning:
- “being away from home,” “no one at home,” “the house is unattended.”
留守にする is a set phrase:
- X を留守にする = “leave X unattended / be away from X (usually a home/office).”

So 明日は一日家を留守にします focuses on the state of the house:
> The house will be in a no-one-home state all day tomorrow.

It’s not just “I will go out sometime”; it implies:

You won’t be at home at all or for a long stretch.
The home will be empty/unattended during that time.

For simply “I will go out,” you’d say things like:

出かけます – I’ll go out.
外出します – I’ll go out / go outside (more formal).

Those don’t emphasize the house being left empty.

Why is it 一日 here and not 一日中? Is there a difference?

Both are possible, but the nuance is slightly different.

一日 (いちにち)
Often means “(for) the whole day” or “all day (long)” in contexts like this.
明日は一日家を留守にします = “Tomorrow I’ll be away from home all day.”
一日中 (いちにちじゅう)
Much more explicitly “throughout the entire day / all day long.”
明日は一日中家を留守にします sounds a bit stronger or clearer that from morning to night you won’t be home.

In everyday speech, 一日 is often enough to imply “all day” when the context is like this sentence. 一日中 just stresses it more.

Can 一日 here also mean “the first day of the month” (ついたち)? How do I know?

No, in this sentence it does not mean “the 1st (of the month).”

As a date: 一日 (ついたち) = “the first day of the month.”
As a duration: 一日 (いちにち) = “one day / all day.”

Here it clearly functions as a duration phrase: “for one day / all day,” modifying 家を留守にします.
Also, 明日 already gives the time (“tomorrow”), so reading 一日 as another date would be strange. Native speakers automatically parse this as いちにち.

Why is it 明日は with は instead of 明日が or something else?

は here marks 明日 as the topic of the sentence:

明日は = “As for tomorrow,” / “Tomorrow, …”

This is a very natural way to start a sentence about what will happen tomorrow.
Using が (明日が) would be unnatural here, because が marks the grammatical subject, and 明日 is not the “doer” of an action; it’s a time expression.

In Japanese, time expressions are often either:

left with no particle: 明日一日家を留守にします。
or made topic with は: 明日は一日家を留守にします。

So 明日は is just “talking about tomorrow” and then saying what will be true about it.

Why is there no particle after 一日 (like 一日に)? How is 一日 working grammatically?

一日 here is a time duration expression functioning as an adverbial (it modifies the verb phrase).

In Japanese, many time-related words are used without particles:

三時間待ちました。 – I waited three hours.
毎日日本語を勉強します。 – I study Japanese every day.
一週間旅行します。 – I’ll travel for a week.

Similarly:

一日家を留守にします。 – I’ll be away from home for one day / all day.

Adding に (一日に) would change the meaning to “on one day / per day,” which doesn’t fit here. So the bare 一日 is correct as a duration adverb.

What is the difference between 留守にする, 留守です, and 留守をする?

They are related but used a bit differently.

留守です
- Describes a state: “(someone) is not home,” “(they) are away.”
- You’d often see it from the viewpoint of someone else about you/them:
  - 田中さんは今、留守です。 – Mr. Tanaka is not home now.
留守にする
- Focuses on making something be in that state (leaving home empty):
  - 明日は一日家を留守にします。 – I’ll leave the house unattended all day tomorrow.
留守をする
- A bit more formal/old-fashioned or used in set phrases:
  - 父が留守をしています。 – My father is away (from home); he is out.
- Can sometimes mean “to stay home while others go out,” depending on context, but usage is limited and nuanced.

In modern, everyday speech for “I’ll be out / away from home,” 留守にする (especially 家を留守にする) is the most common pattern.

Could I say 家に留守にします instead of 家を留守にします?

No, 家に留守にします is ungrammatical.

The fixed pattern is:

X を留守にする

You cannot put に after 家 in this expression.
If you use に, it has to attach directly to 留守 in its adjective-like use:

家は留守です。 – The house is empty / no one is home.
家は留守になっています。 – The house is in a state of being unattended.

But for “I (will) leave the house unattended / be away from home,” you need:

家を留守にする (not 家に留守にする).

Can I use うち instead of 家? Is there any nuance difference?

Yes, you can say:

明日は一日うちを留守にします。

Both 家 (いえ) and うち can mean “home,” but:

家 is a bit more neutral/physical, like “the house (as a building).”
うち often feels more personal, like “my place / our home,” and is common in casual speech.

In this sentence, both sound natural. うちを留守にします is slightly more colloquial and “from the speaker’s viewpoint,” but the meaning is basically the same.

Could I change the word order, like 明日は家を一日留守にします? Is that still correct?

Yes, that is still grammatical:

明日は一日家を留守にします。
明日は家を一日留守にします。

Both are understandable. The default, most natural order is usually:

[Time] [Duration] [Object] [Verb] → 明日は一日家を留守にします。

Putting 一日 right after 明日 groups the time expressions together and sounds smoother.
明日は家を一日留守にします is not wrong, but most learners are encouraged to stick with the more common 明日は一日家を留守にします order.

Does します here mean future tense (“will”)? Why not a special future form?

Japanese doesn’t have a separate future tense. The same non-past form (します) can mean:

present habitual: “I do …”
future: “I will do …”

Which one it is depends on context. Here, 明日 (“tomorrow”) clearly points to the future, so:

家を留守にします = “I will be away from home / I will leave the house unattended.”

If you removed 明日, 家を留守にします would often be understood as a plan / near future (“I’m going to be away from home”), especially in conversation. Context and time expressions do the work that English will does.