Dear Jyoti,
Most of the prominent IT companies do export of their software and deal with their foreign clients and have to attend to their SW related queries only during their clients' working period. Since they are locatd mostly in USA, UK, European countries and Australia, and they observe 5 days' week, that can be the main reason for them to keep Saturday as an off day, as the same could prove as a day of slackness without any need arising out of their clients on that day. But usually they work for more hours on other working days.