Flannelette Time

As my love has already pointed out I am a recent convert to the way of the flannelette pyjama. I can’t believe I’ve waited this long to be seduced by the flannelettish goodness. My one question though – what is the earliest one can pop in to their pyjamas for a nice night in front of the telly or reading, or is it more a function of the time, date and outside temperature?