My dad mentions my brother at Christmas dinner but that's it really. I think the best way to remember someone is to live your life as well as possible and keep them as a happy memory and to honour them with your own life. Did your dad have a particular tradition associated with him, like a favourite Christmassy food or a present he'd always receive? Cos you could incorporate that, like if your dad forever got a pair of socks at Christmas you could all gift each other a pair as a way of remembering, but being happy about it, you know? Don't lose out of traditions that your dad would have pushed for, like maybe playing a particular game or whatever. And if there was nothing that stood out as his at Christmas, perhaps light a candle for him before Christmas dinner and have a toast to his life.