"You're going to have an interesting year,"...
-Bill Weasley, seeing off Harry, Hermione, and his school-aged siblings (GF11)
Harry, Hermione, and most of the Weasleys travel by way of three Muggle taxis from the Burrow to King’s Cross, crammed in with their luggage and overexcited pets. After the group reaches Platform Nine and Three-Quarters, Mrs. Weasley, Bill, and Charlie hint at the surprise tournament awaiting the students at Hogwarts and send Fred, George, Ron, Ginny, Hermione, and Harry off. Aboard the train, Harry, Ron, and Hermione overhear Draco Malfoy praising Durmstrang. They also seen Dean, Seamus, and Neville, and the boys discuss the Quidditch World Cup. Malfoy interrupts, accompanied by Crabbe and Goyle. Malfoy mocks Ron, and makes it clear that he (Malfoy) knows all about the big secret waiting at school. The rain, which has been pouring since the journey began, is coming down stronger than ever when the students get off the train and head for the castle. (GF11)
We know that they travel to Hogwarts on a Monday because the evening before is identified as a Sunday:
"'It's been an absolute uproar,' Percy told them importantly the Sunday evening before they were due to return to Hogwarts." (GF11)
It's clear from the story that the rest of the events described of that night are taking place the evening before they leave.
Alchemical principles factor heavily into Harry's story. The middle of the three phases of alchemy is the albedo phase, which is about purification and, thus, water (source: www.harrypotterforseekers.com). The emphasis on the unrelenting rain that persists throughout the journey back to Hogwarts to start a new term in the central book of the series corresponds to the alchemical process. -BB