This is a sentence which uses extraposition from noun phrase movement. Often when a noun phrase (NP) has a long relative clause, or other modifier, that modifier gets moved out of the noun phrase and appears at the end of the sentence. This is particularly likely when the rest of verb phrase is very short (here it is just last week). Here is another example:
- We fired three people last week who had been stealing parrots.
In the sentence above, the phrase who had been stealing parrots has been moved to the end of the sentence. The canonical word order would be:
- We fired [three people who had been stealing parrots] last week.
The longer the modifier and the shorter the verb phrase the more likely this is to happen. It very often happens, therefore, with intransitive verbs like COME.
- An inspector came in from the Inland Revenue.
Notice that the sentence above probably does not mean that they came in from the Inland Revenue!
The Original Poster's sentence
At last the day came when everyone except the Kelveys had seen the doll's house.
The Original Poster's sentence is slightly confusing because of the non-canonical word order. Part of the subject phrase has been moved to the end of the sentence. If we keep the subject intact, as one phrase, we get:
- At last [the day when everyone except the Kelveys had seen the doll's house] came.
Although this is the canonical word order, it is a bit clumsy because the subject is so long. It is, however, easier to see how the grammar works!
Note: In case you want to Google it, extraposition from noun phrase movement is often written: extraposition from NP