holiday allowance

my new contract specifies that i have 20 days holiday a year (i.e. 4 weeks). but that's 20 days from January to December, and my contract starting May 1st, i end up with just 13 days, which happens to be exactly the time i'm taking over summer. lucky me. unluckily, i'd planned to take a day in Sept and another couple in December, but it looks like that won't be possible anymore...