Filing of VAT/CST return is mandatory if one is dealing in trading of goods and is a registered dealer under State VAT and CST laws. If a dealer is only dealing in trading of goods within a particular State (where registration is sought) then only return under State VAT law is to be filed and no need arises to file a CST return but if trading of goods is effected inter-state i.e. from one state to another, then the requirement for CST registration arises and returns need to be filed.
Returns can be filed either monthly or quarterly and that frequency is determined basis the turnover of the registered concern.. Due dates for filing VAT/CST returns varies from State to State since every State has its own law implemented for its territory. To check the last date of filing return for your State, kindly refer to this list.