Head bath -Is it correct?

Yes, but ...

"Head bath" is an Indian-English usage. American and British English speakers would say "I wash my hair every Saturday".

In researching this answer,  I also found that Indian-English includes "hairwash" and "hairwashing" as verbs. While neither appears in US or UK dictionaries (even OED), the addition to the language makes sense. I also found that "head bath" developed to serve as a broader, more encompassing term than hairwash. After all, a bald man cannot wash his hair, but he can still give his head a bath.