The journey through the Nine Worlds will start with Midgard. There are a lot of differing opinions as to what and where Midgard actually is. One opinion is that it is actually a reference to our world, our material plane here. However, although our world here does have an "astral" body, just as we do, that is not Midgard.
Midgard is a world that spins around the World Tree that is Yggdrasil. It is the one of the Nine Worlds that lies the closest to our own, and as such it's the easiest to get into. Beginning journeyers would do well to start with moving themselves into Midgard before moving to any of the other worlds. Actually, that's often exactly what happens, if only unconsciously - the journeyer's thoughts move their body in the direction of Yggdrasil, and it's Midgard they get pulled into, sometimes only for a few seconds before moving to another of the worlds. The concept of Midgard as being central and close is so strong that much of the lore claims that it is the central world that all other revolve around.
Some journeyers experienced it that way, or haven't looked critically enough to see any differently. Others have noticed Midgard as being simply one world in a line, spiralling around Yggdrasil at about mid-level, and somewhat to the east. This differentiation varies depending on whether journeyers are seeing the worlds organized into a spiral or into three flat layers. The denizens of the other eight worlds do not see Midgard as particularly more important than any other world, and less so than some. They also do not see it as physically central, so it may be that this perception is largely our own Earth-centric and thus Midgard-centric view.
Midgard itself seems to be similar in nature to our world many centuries ago. There is some speculation that there may have been a time when the two worlds were joined even closer than they are today, and that the boundaries between astral-Earth and Midgard, or even physical-Earth and Midgard, sometimes blurred and folk could wander back and forth, on purpose or accidentally. As the split between the two seems to be that our world is so much more physical/material than the Nine, it may be that many centuries ago, that split was not as pronounced, and our world has changed some since then. This gives rise to the idea that the humanfolk of Midgard actually once came from our world, when the two were more joined than they are today. However, there is almost no way to probe further than speculation into these matters.
As the sister-world to our world, Midgard's seasons are lined up perfectly with ours - or, more specifically, those of the northern half of our northern hemisphere. This may be why Midgard is so much more accessible to us. Day length varies depending on time of year and where you are in Midgard. The continent is about the longitude of Europe from the Mediterranean to the Arctic Circle, and in latitude about as wide as Eurasia, so day length will vary accordingly.
To be continued.