Maybe it happens after a big victory at Hogwarts. There's a huge celebration with music and dancing. Harry and Hermione find themselves alone for a moment on the balcony overlooking the festivities. The atmosphere is filled with joy and a sense of new beginnings. Harry takes Hermione's hand, and without a word, they lean in and share their first kiss, a celebration of their friendship evolving into something more.
In a time - travel scenario. Harry and Hermione go back in time to change an important event. After successfully achieving their goal, they are in a state of relief and wonder. Standing in the quiet of the past, away from the chaos they just averted, they are overcome with their feelings for each other. And that's when their first kiss occurs, as if time has stopped just for them.