Hello
If you are looking for a BRF+ approach then refer to blog below
Solving impossible date & time calculations... | SCN
OR
If you are interested to use a FM in background workflow task then use following FM to get holiday info.
HOLIDAY_CHECK_AND_GET_INFO - Check whether or not a date is a holiday. Give the function
a date, and a holiday calendar, and you can determine if the date is a holiday using parameter HOLIDAY_FOUND.
Regards
Sandy