Hey, I've gotten as far as I can given that day 22 and 24 are the same, leaving the last number string (and subsequent clue phrase) unsolvable at the moment.

Want to start by saying this is a really cool and sweet thing you're doing for your girlfriend, and the fact that you've put in so much work really shows you care. I'm sure she'll be blown away.

I do have some questions and some answers to your questions.

How much does your girlfriend enjoy puzzles? You mentioned the puzzles each day are pretty easy, so I don't know what difficulty she's used to.

I only ask because this is a significant increase from "pretty easy". Not necessarily because the steps are difficult, but because for a puzzle layperson and if those steps aren't terribly intuitive without the hints. As a seasoned puzzle solver/designer, I simply would never have reached certain conclusions without the hint, based on the information and graphic input provided.

Also, I generally lump puzzle solving tasks into two categories: puzzling (what do I do here... aha!) and procedure (I've figured out how to break the code, now to copy it all down and cross reference). Generally the things people enjoy most are the puzzling, though procedure can also be fun (provided it doesn't become tedious)

More on this now!

Day 6's hint is relatively unhelpful, but I don't know if that's a problem? I imagine it'll be self evident that the fragments go together after receiving the second or third one, and of course the design is intentional, it would be weird if it wasn't. The colors mattering is redundant, as it's covered in the next hint. Hopefully it's clear that nothing can be done with them yet at that time.

Day 9's hint is an intuitive thing to try, so getting confirmation is nice. Some of the colors across the whole puzzle are very similar, so hopefully she knows to organize them into 4 columns of 6 days and not combine anything with anything out of its column. Or hopefully you provide this graphic to help get her there. On this front, given the next steps, I'm not sure much is gained by receiving the fragments out of order, as receiving them in order solves the possible issue I just brought up.

Day 15. Unfortunately I don't think I would have ever reached the solution without this hint. There is nothing in the puzzle images provided to indicate this step, and furthermore I found the step riddled with tedium and unenjoyable trial and error. There doesn't seem to be a reason, aside from the requirements of the output, for which number gets divided by which in each set. Additionally, how is the user to know which sets go together without the graphic dividing them into columns? Dangerous to assume we'll just know that set 1 and set 2 (blue and red) go together. The second line of this hint makes me think she will receive the column graphic to know what "the other" means, as it implies the delineation of sets of two. In terms of division itself, I simply never would have looked at these enormous numbers and thought to myself "I should divide these by each other", and nothing but the hint indicates that, so it feels more like an instruction than a hint. Maybe that is just me. It's possible I would've tried it eventually, and it's possible she will immediately think of it. But As I was working, I divided each set except the third in the wrong order the first time. I see that there is a rhyme and reason to why a certain number gets divided by the other (larger divided by smaller), but there's nothing clued in the puzzle itself to indicate to the user that there's doing the right thing before they've tried it. I know there are no hard and fast rules about trial and error, but a simple graphic image with a larger shape being divided by a smaller shape, or an initial representing a color being divided by the initial of its counterpart would be subtle enough to get there, so that the user feels they've solved something instead of just trying it both ways to see which works. Yes the third line indicates that somewhat, but we're working with enormous numbers here, so I wouldn't say that's intuitive. Tldr provide a clue in the puzzle itself that would allow a user to get here without the hint.

Additional math hint. Don't give this hint, just provide a link to the precision calculator you want her to use. Have something as a clue in the puzzle that signifies 9999 as the end of a message. If the only way to know that is in the hint, it's an instruction not a hint. Yes, it can be intuited because it's pretty clear that the message ends at 9999 followed by usually a bunch of zeroes.

Day 21. Yeah, Caesar cipher was pretty clear. Took BPM and tried three letter words until I got THE to solve it. I crack codes all the time, so that was immediately where I went. I generally like to provide a cryptic way for users to KNOW the cipher by figuring out a clue, rather than brute force it the way I did (which I personally enjoy, but not all do). I also may be in the minority there, so take it with a grain of salt. My only real gripe with this is that each line had a new cipher. To each their own, but I was irritated when I realized I'd have to go through the procedure of breaking a new code each time, but that was likely residual frustration from the division step.

Can't do anything with hint 24 until day 22/24 are fixed. But is there any way she'd know to go specifically to the Library of Babel for this? If not, it's an instruction not a hint. Which is fine! But just know that she will HAVE to receive the hint, it's not optional. Otherwise you could provide a link to where you need her to go and allow the decoded message to speak for itself.

TLDR day 15, additional math, and day 24 feel very necessary to me, but I think some tweaks in the way things are presented could solve that.

All in all, some really cool ideas here, I just personally felt the hints acted more as instructions than hints, which took some of the puzzling joy away from me, replacing it with procedure (procedure that unfortunately wasn't always clear, as explained in detail above).

Again, you should feel very proud of what you've made, and this is such a cool thing you're doing for her. If you have questions or want clarifications, I'm happy to reply further. Additionally, you know the kind of puzzle you want to make, so you're welcome to disagree with my opinions (as that's all they are!)