What are your theories for the lost day after Lily and James died because you just seem like the kind of person to have some
So my real theory is that JKR simply screwed up the timeline a little. But if we're trying to make it work, my headcanon is as follows:
Evening of October 31st: Voldemort goes to Godric's Hallow. It can't have been too late because there were trick-or-treaters outside and Harry was still awake, so let's say 7pm. The spell backfires, Voldemort is left in some sort of shriveling form.
We know a large portion of the house was blasted away, but of course it may have taken some time for people to realize what happened due to various enchantments put on the house, or they might not have wanted/been able to get too close. It would initially be assumed the entire family was dead.
Slowly, as it got later at night, it becomes clear that Harry was alive and well. Dumbledore receives word of their deaths/Harry's survival via charms on the house or contacts on the ground. Dumbledore dispatches Hagrid to retrieve Harry.
Still on the night of the 31st or in the wee hours of the 1st, Sirius hears about what happened through his own contacts/methods, and of course immediately proceeds to the Potters. Sirius arrives just after Hagrid and gives him the motorbike. He sets off to find Peter.
Meanwhile, Dumbledore having ascertained via Hagrid that Harry is immediately safe, has other matters to attend to. All he knows is that L/J is dead, Harry is not, and there's a hole in the side of their house. It would not be immediately clear that it was Harry's doing or if Voldemort had simply gone underground. So he's spending the first few hours trying to piece together what happened and who is responsible. By the early morning Dumbledore and perhaps others on the scene have started to surmise that Voldemort is truly gone.
But separately, news is beginning to leak out from Godric's Hallow. Voldemort's Death Eaters are starting to learn of their leaders disappearance, and making plans to escape, cover their tracks, to mount a last stand, and/or to track him down. The Ministry and Magical Law Enforcement is pressing their advantage. Given Dumbledore's position and connections, it's highly likely he was involved in various conversations with the MLE/Minister.
And then separately, Dumbledore is now working on a plan for Harry, which involves confirming somehow that Lily's magic is what protected him, deciding to place him with Petunia, fending off any Ministry attempts to place Harry elsewhere, and tracking down her residence.
Around midday on the 1st, Sirius catches up with Pettigrew and "kills" him, surrounded by witnesses.
Meanwhile, Dumbledore has decided to place Harry with the Dursleys. But for tactical reasons he wants to leave him on the doorstep rather than risk them refusing, so he arranges for Hagrid to meet them late that night.
But then of course the question is what Hagrid was doing. My personal headcanon is that he was simply waiting around somewhere secure but nearby at somewhere like Bathilda's until late in the evening, then left via motorbike to meet with Dumbledore at the arranged time. It's admittedly a little odd to just leave Hagrid waiting like that but a) it's perfectly logical that Dumbledore would need more than a few hours to make arrangements b) having sent Hagrid to retrieve Harry, I don't think Dumbledore would want him handed off to anyone else, especially not on such a chaotic day.