fill in the correct form, please

[Tukanja]

Please fill in (out ) the right response.

I'm sure that happened ____________ we spent in Switzerland.

1. Christmas
2. at Christmas
3. in the Christmas
4. the Christmas

An explanation's needed.

Thanks

[Joe]

4

[Tukanja]

Does this sentence mean as below

I am sure that that happened during the Christmas (which, that) we spent in Switzerland.

Thanks

[Joe]

Yes