Если завтра выходной, я хочу доспать хотя бы до восьми.

Russian often leaves out to be in the present tense, and this pattern is also very common in short statements about schedules, dates, and calendar facts.

So:

• Завтра выходной = Tomorrow is a day off
• Если завтра выходной = If tomorrow is a day off

You can also say Если завтра будет выходной. That version is a bit more explicit, but the version without будет is very natural and common.

Here выходной means a day off.

Literally, it comes from выходной день:

• выходной день = day off
• выходной by itself = day off (with день understood)

So in everyday Russian, people very often just say выходной without день.

In this sentence, если means if and introduces a condition:

• Если завтра выходной, я хочу... = If tomorrow is a day off, I want...

That means the speaker’s wish depends on whether tomorrow is a day off.

In other contexts, если can sometimes feel close to whenever or appear in fixed expressions, but for learners, if is the right understanding here.

Доспать is not just to sleep. It means something like:

• to sleep a bit more
• to finish sleeping
• to catch up on the rest of one’s sleep

The prefix до- often gives the idea of reaching an endpoint or finishing something.

So:

• спать = to sleep
• доспать = to sleep some more / finish sleeping

In this sentence, я хочу доспать хотя бы до восьми suggests the speaker wants to keep sleeping until eight, probably because they have not had enough sleep.

Because it follows хочу:

• я хочу доспать = I want to sleep some more / I want to sleep until...

After хотеть (to want), Russian normally uses an infinitive for the action wanted:

• Я хочу спать = I want to sleep
• Я хочу поесть = I want to eat
• Я хочу доспать = I want to sleep some more

Yes, absolutely. After хочу, Russian can use either imperfective or perfective, depending on the meaning.

Here доспать is perfective because the speaker means a completed action with a clear limit: sleeping up to a certain point.

Compare:

• хочу спать = I want to sleep / I feel sleepy
• хочу поспать = I want to sleep for a while
• хочу доспать до восьми = I want to sleep on until eight

So the perfective here helps show a bounded result: sleeping until that time.

Хотя бы means at least.

So:

• до восьми = until eight
• хотя бы до восьми = until at least eight

It shows a minimum acceptable result. The speaker would like to sleep until eight, and possibly even later, but eight is the minimum they are hoping for.

Because до means until/up to, while в means at.

Compare:

• спать до восьми = to sleep until eight
• в восемь = at eight o’clock

So if the idea is continuing to sleep up to that time, Russian uses до + Genitive:

• до восьми

If you said в восемь, that would point to the exact time at eight, not the endpoint of sleeping.

Because the preposition до requires the genitive case.

So:

• nominative: восемь
• genitive: восьми

That is why Russian says:

• до восьми = until eight

This is a very common pattern:

• до утра = until morning
• до вечера = until evening
• до понедельника = until Monday
• до восьми = until eight

Because Если завтра выходной is a subordinate clause introduced by если, and Russian separates it from the main clause with a comma.

Structure:

• Если завтра выходной, = subordinate clause
• я хочу доспать хотя бы до восьми. = main clause

This is standard Russian punctuation.

It could be omitted.

Russian often drops personal pronouns when the verb already makes the subject clear:

• Я хочу доспать хотя бы до восьми
• Хочу доспать хотя бы до восьми

Both are natural.

Including я can make the sentence a little clearer or slightly more emphatic, but it is not required.

Yes, you could, but the nuance changes a bit.

• поспать = sleep for a while
• доспать = sleep some more / finish sleeping

So:

• Я хочу поспать хотя бы до восьми = more neutral, just I want to sleep until at least eight
• Я хочу доспать хотя бы до восьми = suggests I want to get the rest of my sleep / keep sleeping until at least eight

In this sentence, доспать feels especially natural if the speaker is tired and wants extra sleep.

The sentence is natural as written, but Russian word order is flexible.

Original:

• Если завтра выходной, я хочу доспать хотя бы до восьми.

Possible variation:

• Если завтра выходной, хочу доспать хотя бы до восьми.
• Я хочу хотя бы до восьми доспать.
  This is possible, but less neutral.

The original word order is a very normal, neutral way to say it. In Russian, changes in word order usually affect emphasis more than core meaning.