The coldest I've experienced up there was 11°C in the summer of 2012. I was there with my then girlfriend and some of her relatives at the end of Holy Week of that year.
Anyway, my mother's side of the family is from Pangasinan and Ilocos Sur. But I've never been up to Baguio with my family, only with other relatives.
And one thing I should always remember is that Holy Week is not a nice time to visit as it is most lowlanders and Manileños favorite time to go there. Christmas, however, is definitely an exception.
It is not always nice to visit Baguio during peak holiday seasons, including Christmas, because of bad traffic. Though, that is part of the excitement! 😄