Mae’n ofynnol i awdurdodau lleol hysbysu Gweinidogion Cymru o ddyddiadau’r tymhorau a bennir ar gyfer pob ysgol a gynhelir (gan gynnwys ysgolion gwirfoddol a gynorthwyir ac ysgolion sefydledig) yn eu hardaloedd erbyn diwrnod gwaith olaf mis Awst. Os na ellir cytuno, er gwaethaf pob ymdrech i wneud hynny, mae Deddf Addysg 2002 yn rhoi’r pŵer i Weinidogion Cymru roi cyfarwyddyd i awdurdodau lleol a chyrff llywodraethu perthnasol ar ddyddiadau eu tymhorau, er mwyn cysoni dyddiadau tymhorau ledled Cymru.
Local authorities are required to inform the Welsh Ministers of the term dates set for all maintained schools (including voluntary aided and foundations schools) within their respective areas by the final working day of August.   If, despite best efforts, consensus is not achieved the Education Act 2002 gives the Welsh Ministers the power to direct local authorities and relevant governing bodies on what their term dates must be, so that term dates are harmonised across Wales.