What Sateesh and Helen say is correct. Legally, there is no need to give compensatory off when statutory or agreed-upon holidays fall on weekly offs. This does not only occur on Christmas, but on other holidays falling on weekly offs, and legally speaking, there is no need to compensate for such days.
However, each company is free to follow its own policy on such occasions, and there are companies who allow compensatory offs. But if they do so, it must be on all similar occasions, not only on 25th December.
Also, when they do give such offs, they should clearly indicate that this is as per their own policy, and not as followed by the majority of companies, because this is not the case.
HR managers must be free to opt for either screnario depending upon their own company policy, and not on what others say or do.
Jeroo