In message , Dave Cornwell
writes
Personally, in January I would historically not consider it a cold
spell till daily maxima fell to around 2-3C or less for more than 3
consecutive days (roughly, in my own mind). Night time frosts might
come into play as well.

That seems about right to me.
--
I'm not paid to implement the recognition of irony.
(Taken, with the author's permission, from a LiveJournal post)