Hi,
Refer below link.
Calculating Number of Working Days in Query Level
Before that In your BW system goto tcode- RSA1-> Select your source system-> Right click on this & select transfer global settings-> Select factory calendar & execute-> factory calendr will be updated in BW system.
you can check this in table- TFACD(Both in R/3 & BW system)
Use the Factory Calendar ID configured in your system.
Regards,
Sucheta