This might sound like backhanded praise, but this is very good for both a first game AND a first platformer. Most developer's first platformers tend to be... quite bad, for a lot of reasons, but you managed to avoid most of the severe pitfalls.
The controls are especially good for a first platformer. There's a little bit of momentum, but it never renders the player a slippery mess. I especially like the interval between the short hop and long jump; it's just wide enough to make jumps feel player-controlled, but not so wide as to render short presses useless.
I noticed that wall-jumping is possible by jumping into a wall and turning around. I really liked this feature; it could've been mentioned in the instructions.
All the levels were fun, with a nice middle-ground of difficulty. They were never cake-walks, but even the more difficult platforming challenges (like the disappearing platforms or the sawblade tunnel) never felt outright unfair. Maybe the small platforms could be a smidge wider.
I think the most "unfair" design element is the very short invincibility interval after being damaged; it's quite easy to get stuck inside of saws or enemies and get shredded into pulp in a few instants.
The day/night mechanic helps make the short game feel more like a journey. It actually took me a couple of minutes to notice that nighttime made the enemies bigger, which was a neat detail.
I think certain background elements could be better distinguished; a lot of the pastel colors in the first level especially tended to blend together. Additionally, the art clash between the edgeless vector graphics and the upscaled pixel art graphics was a bit distracting. That could be ironed out a bit.
All in all, great job!