What's the weather like in Hong Kong during Christmas? How many clothes and pants can I wear?

As for the weather, you can go to the Hong Kong Observatory, but the forecast is only seven days. You asked too early.

But I don't think a long-sleeved T+ skirt will work unless you are cold-resistant. . .

There seems to be no fireworks at Christmas in Hong Kong, only New Year's Day and National Day.

As for special activities, there should be many people celebrating Christmas in Lan Kwai Fong.

There are Tsim Sha Tsui seaside lights, Victoria Harbour and so on.