Summer brought a cold twist to Hogwarts' corridors—Snape took over right after Dumbledore's death. In the pages of 'Half-Blood Prince' the change is immediate: after the tragedy on the Astronomy Tower at the end of that book, Severus Snape is installed as Headmaster. That effectively happens in the summer of 1997, once the Ministry and Voldemort's allies start reshaping the school to their ends.
He presides over the 1997–1998 school year under the shadow of Death Eater influence. It isn't a peaceful tenure: rules tighten, students whisper, and his authority is both feared and resented. Later, in 'Deathly Hallows', the full horror of that era and the consequences of his leadership—culminating in his death during the Battle of Hogwarts—are revealed. For me, Snape’s headmastership is this wildly tragic, morally ambiguous chapter: it flips Hogwarts from sanctuary to battleground and shows how power can warp even familiar places.