It’s amazing how vacation (holiday to the British) is handled in Europe. In the UK, a fulltime employee is allocated 28 days off. In some countries (like Austria, Denmark, and Sweden), this number is over 35 days. In addition, most companies will allow their employees to carry-over holiday to the next year. If you include national holidays, then most folks get well over six weeks vacation.
Compared to the US (two weeks, maybe three/ four weeks with holidays and tenure), the European model is very generous. From what I gather most folks take off two or more weeks in the summer, around two weeks at Christmas/ New Years, and they spot fill-in with long weekends in the Spring and Fall. Business comes to a halt in August (for the Summer holiday) and at the end of December (like most places in the states).
Allocating for vacation, in Europe, varies the planning of project work/ deployments and interacting with customers and groups in different locations. Likewise, resource utilization can change from country to country (especially when working with folks in India). So, assuming everyone works at 80% throughout the year except Christmas through New Years is a recipe for incorrect estimates.