How do you calculate a worker's entitlement to annual leave where their hours increase or decrease as their contract varies, e.g. they go from part-time to full-time hours or vice versa? Each period must be separately calculated, according to the CJEU:

"... as regards the accrual of entitlement to paid annual leave, it is necessary to distinguish periods during which the worker worked according to different work patterns, the number of units of annual leave accumulated in relation to the number of units worked to be calculated for each period separately... where, as in the case in the main proceedings, the worker, after accumulating rights to paid annual leave during a period of part-time work, increases the number of hours worked and moves to full-time work... the number of units of annual leave accumulated in relation to the number of hours worked must be calculated separately for each period... In a situation such as that at issue in the main proceedings, EU law therefore requires a new calculation of rights to paid annual leave to be performed only for the period of work during which the worker increased the number of hours worked. The units of paid annual leave already taken during the period of part-time work which exceeded the right to paid annual leave accumulated during that period must be deducted from the rights newly accumulated during the period of work in which the worker increased the number of hours worked."
http://curia.europa.eu/juris/document/document.jsf?text=&docid=171284&pageIndex=0&doclang=EN&mode=req&dir=&occ=first&part=1&cid=535755