WORKDAY(date,days,holidays,weekend)
date: date serial value
days: number of days to add
holidays: array of holidays
weekend: array of 0s and 1s, indicating whether a weekday (S, M, T, W, T, F, S) is on the weekend, defaults to {1,0,0,0,0,0,1}
WORKDAY adjusts date by days skipping over weekends and holidays in the process.
days may be negative. If an entry of weekend is non-zero, the corresponding weekday is not a work day.
This function is Excel compatible if the last argument is omitted.
This function is OpenFormula compatible.