Kun pyykkivuoro, saunavuoro ja siivouspäivä ovat samassa viikossa, kirjoitan ne heti kalenteriin, etten unohda mitään.

Here kun means when. It introduces a subordinate clause that gives the situation or time frame for the main action.

• Kun ... ovat samassa viikossa = When ... are in the same week
• Main clause: kirjoitan ne heti kalenteriin = I write them in my calendar right away

In other contexts, kun can also mean as, while, or sometimes even because, but here when is the natural meaning.

Each noun is singular because each one refers to one event/day/slot:

• pyykkivuoro = a laundry turn / laundry slot
• saunavuoro = a sauna turn / sauna slot
• siivouspäivä = a cleaning day

Finnish often lists singular nouns like this if you mean one of each. English does the same:

• Laundry day, sauna turn, and cleaning day are all in the same week

The whole subject is plural because it contains three items together, so the verb is plural: ovat.

Because the subject is made up of three coordinated nouns:

• pyykkivuoro
• saunavuoro
• siivouspäivä

Together they form a plural subject, so Finnish uses the plural verb ovat = are.

Compare:

• Saunavuoro on maanantaina. = The sauna turn is on Monday.
• Pyykkivuoro ja saunavuoro ovat samassa viikossa. = The laundry turn and sauna turn are in the same week.

Samassa viikossa means in the same week.

It consists of:

• sama = same
• samassa = in the same
• viikko = week
• viikossa = in the week

Both words are in the inessive case, which often expresses in.

So:

• samassa viikossa = in the same week

This is very typical Finnish case agreement: the adjective and noun usually take the same case.

Because kalenteriin is the illative case, which often means into or onto, depending on the context.

• kirjoittaa kalenteriin = to write into / in one’s calendar
• kalenterissa would mean in the calendar as a location, not the destination of the action

So the idea is:

• I put the information into the calendar.

This is a common Finnish pattern:

• laittaa laukkuun = put into the bag
• merkitä kalenteriin = mark in the calendar
• kirjoittaa vihkoon = write in/into the notebook

Ne means them, and it refers back to the three things mentioned earlier:

• pyykkivuoro
• saunavuoro
• siivouspäivä

So:

• kirjoitan ne heti kalenteriin = I write them in the calendar right away

In everyday Finnish, ne is very commonly used for things, and also often for people in spoken language. Here it clearly means those scheduled items.

Etten is a shortened form of että en.

• että = that
• en = I do not

So:

• etten unohda mitään = so that I don’t forget anything

This kind of contraction is very common in Finnish writing:

• etten = että en
• ettet = että et
• ettei = että ei
• ettemme = että emme

In this sentence, it expresses purpose:

• I write them down right away so that I won’t forget anything.

Because the clause is negative:

• en unohda mitään = I don’t forget anything

In Finnish, negative clauses often use the partitive in places where English uses words like anything.

Compare:

• Unohdan jotain. = I forget something.
• En unohda mitään. = I don’t forget anything.

So mitään is the natural form after negation.

Because Finnish negative verbs work differently from English.

In a negative sentence, the personal ending goes on the negative verb, and the main verb appears in a special shorter form.

• unohdan = I forget
• en unohda = I do not forget

So in:

• etten unohda mitään

the I is already expressed by en, so the main verb stays in the negative form unohda.

This is a basic Finnish pattern:

• minä kirjoitan = I write
• minä en kirjoita = I do not write

Heti means right away, immediately, or at once.

So:

• kirjoitan ne heti kalenteriin = I write them in the calendar right away

It emphasizes that the speaker does it without delay, probably because there are several things to remember in the same week.

Because the sentence begins with a subordinate clause introduced by kun:

• Kun ... ovat samassa viikossa, = When ... are in the same week,

After that comes the main clause:

• kirjoitan ne heti kalenteriin

Finnish uses a comma between these clauses, just as English usually does when a when-clause comes first.

There is also a comma before etten, because that begins another subordinate clause:

• ..., etten unohda mitään.

Yes. Finnish uses compound words very often.

Here:

• pyykki = laundry
• vuoro = turn / shift / slot
  → pyykkivuoro = laundry slot / laundry turn

And:

• sauna = sauna
• vuoro = turn / slot
  → saunavuoro = sauna slot / sauna turn

Also:

• siivous = cleaning
• päivä = day
  → siivouspäivä = cleaning day

For English speakers, compounds are important in Finnish because many ideas that are written as two words in English are often written as one word in Finnish.

Yes, that is a good instinct. In natural English, this idea could be translated in a few similar ways:

• in the same week
• during the same week
• all in one week

The Finnish samassa viikossa literally points to being in the same week, but the practical meaning is that all these scheduled things fall within one week.

Yes, it is natural and idiomatic. It sounds like something someone would say about keeping track of household responsibilities or shared building schedules.

A few things that make it natural:

• common compound nouns: pyykkivuoro, saunavuoro, siivouspäivä
• normal time clause with kun
• natural pronoun reference with ne
• common purpose structure: etten unohda mitään

So it is a good example of everyday written Finnish.