while booking flights.

i'm using my dads credit card.

do i -

a) book flights for fri 22 dec which are approx £50 MORE expensive than the day before.... or
b) save the £50 and go on the thursday, but then have to take an extra days holiday from work, resulting in my holiday days till april being a big fat ZERO?

arrrrrrrrrrrrrrrrrrrrrrrrrrrrrgh