It was July and August. Roman emperors. They put them as early in the cycle as they could without displacing a God.

January/Janus is the god of doorways, thresholds and transitions, and so was the only option for the god of starting a new year. Also more important than we typically consider. There just weren't explicit temples in the same way as others because their domain was every doorway, gate, and transition. When your temple is a giant door people mistake it for a gate. Which it is.