I finally came up with this solution:
={WORKDAY(A2,B2,(DATE(YEAR(A2),$K$20:$K$28,$J$20:$ J$28)))}
whe
A2 = Starting Date
B2 = Number of working days
$K$20:$K$28 = Array of Months
$J$20:$J$28 = Array od Days

so i tried to make a virtual array generating the date from the year
of the cell, days and months(of holidays) provided by the user and
copied it down through the starting days of the projects.
I am not sure if this is OK, so I need your thougths please.