Is Ghusl obligatory once a week?

It is only obligatory to perform a purificatory bath (ghusl) when one is in major ritual impurity. Therefore it might necessitate one to take an obligatory ghusl less then once a week or more. Major ritual impurity is caused by the following: (Reliance of the Traveller, e10.1)

For men:

1) when sperm exits from him;

2) when his penis enters a vagina;

For women:

1) when sexual [s. orgasmic] fluid exits from her;

2) when a penis enters her vagina;

3) after her menstrual period;

4) after her post-natal lochia stops or after a child is born in a dry birth.

The questioner might be referring to bathing on Fridays for the Friday prayer. This ghusl is a sunna and not obligatory. (e11.5)

And Allah knows best.

Shazia