Borrowed days of February
(The ). 12th, 13th and 14th of February, said to be borrowed from January. If these days prove stormy, the year will be favoured with good weather; but if fine, the year will be foul and unfavourable. These three days are called by the Scotch Faoilteach, and hence the word faoilteach means execrable weather.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894