It will be fine. When I worked in a posh restaurant we always ignored the sell by dates anyway, but as long as it's still gassy, it's got to be ok. It tastes best if it's been in the freezer for an hour or so, or in a very cold fridge for a couple of hours. Otherwise it's a bit sweet. I've never heard of anybody being poisoned by Asti Spumante, but it's not favoured by wine snobs, probably because it is sweet. You can make quite a nice cocktail with it using vodka and vermouth. Just chill it and enjoy it - I'll metaphorically drink it with you and a happy Christmas!