How is it that Harry doesn’t see the thestrals at the end of the Fourth year while going back to Hogsmeade but sees them first thing on arriving after the summer?
The question had been bothering me for months, before a student on Hogwarts is Here came to my rescue. She said-
Shellsea Calandra Lark: “JK Rowling addressed this nicely on Pottermore, writing in detail about how it is people get to see Thestrals, and how it isn’t just about “witnessing death” but being deeply impacted or moved by the death, whether that meant being present, or a part of it, etc. So she explains Harry couldn’t see them before the summer, because it was only over the summer that the weight of Cedric’s death started to press in on him and that he was really able to feel it’s impacts on his life. If that doesn’t make sense, go to Pottermore and read JKR’s details, she does a better job than me in explaining!”
So I did. I decided to go and explore Pottermore for myself. This is what I found-
Being able to see Thestrals is a sign that the beholder has witnessed death, and gained an emotional understanding of what death means. It is unsurprising that it took a long time for their significance to be properly understood, because the precise moment when such knowledge dawns varies greatly from person to person. Harry Potter was unable to see Thestrals for years after his mother was killed in front of him, because he was barely out of babyhood when the murder happened, and he had been unable to comprehend his own loss. Even after the death of Cedric Diggory, weeks elapsed before the full import of death’s finality was borne upon him. Only at this point did the Thestrals that pull the carriages from Hogsmeade Station to Hogwarts castle become visible to him. On the other hand, Luna Lovegood, who lost her own mother when she was young, saw Thestrals very soon afterwards because she is intuitive, spiritual and unafraid of the afterlife.
(Source: Thestrals By J. K. Rowling)
That makes sense. Today, I finally sloop in peace.