Bali’s weather is partly influence by australia’s. So when it’s winter there (mid year) then it’s nice and cool too in Bali. Usually starts around june/july to september/october. September/October is the start of rainy season in Indonesia, so you might get rain too. Domestic school holiday is the whole month of june and partial July. Do take into account oz’s winter hol albeit not that long, and pick the time to your liking.

June or July, it's best for summer season