I know how Janus had significance to the Romans and January is the first month for this reason but it seems odd that there's no real difference between January and December from a weather standpoint so wouldn't it have made more sense to make March the first month of the year since that's when spring starts? I know Julius Caesar borrowed a lot from the Egyptian Calendar when designing his so was this just a holdover from the Egyptian calendar being better suited to their climate or was it just completely arbitrary?
It can sometimes feel as though the first of January is in the middle of winter, but the calendar is not established around when the weather feels like a change of seasons (or the year would begin at various times depending on the local climate). It hinges on the winter solstice.
The winter solstice marks the beginning of winter – as a season not how it is perceived. Although it is roughly a week before the 1st of the year, the two weeks on either side of the 1st mark the darkest time of the year in the northern hemisphere. By rights, New Year’s would probably be 21 December, but I think we can forgive a little slippage given how there is not a dramatic difference between 21 December and 1 January in the time of sunrise.
The nice thing about linking the new year to a solstice is that anyone who is observant about the time of sunrise – or better, where the sunrises on the horizon – can determine when the new year is beginning. Changes in the location of sunrise (and sunset) are easily seen the farther one goes to the north (and to the south in the southern hemisphere). This makes it easy to determine when the new year begins – within a day or two – even without a calendar.
This demarcation would have meant less to the Egyptians, being closer to the equator where the solstice is not as easily observed. They focused on other celestial events (especially the rising of Sirius) as calendar devices. There was an alternative European approach linked to the spring equinox (March 21- often recognized as March 25), but that yielded to the winter solstice, which, frankly, is easier to observe.
In addition to u/itsallfolklore's answer, I might add a previous answer from me:
Why is January 1 the first day of the year?
The very short answer is "because that's when the Roman calendar started" but, although it makes logical sense that their year originally started in March and then they moved it to the closest month to the winter solstice...there's actually not much evidence for that. The calendar started in January very early on in Roman history and even the classical Romans we're more familiar with didn't really have any idea why.