Hello guys 🙂
I think tag display in calendar view is a necessary feature in the app. You guys discussed about it, and I wanted to bring the topic back with my opinion, ideas and designs. After all these years, I think it is finally time for this big guy to be implemented 🙂
I think we all agree that the calendar needs some kind of coloring feature to look more attractive visually and better organize the entries. And, as discussed here, I think coloring the entry based on its tags is the key.
Posts that do not have an image all get filled with the same default color, so we could replace this with the tag's color. However, an entry can easily have multiple tags, so there would be the need to pick one of them. Also, apart from the limitation of displaying just 1 tag, this tag visualization would not work when images are attached.
I think we should aspire to display more than 1 tag. I recently came from an app that displays the colors of all the tags in each entry, and I am able to know very quickly what happened that day, whether it was an important day, a stressful day, whether there was a special event, etc. And, personally, I find this extremely useful and valuable.
One solution would be to display them like @GrzegorzCiepka said, replacing the current multi-entry dots with tags and their colors. However, I've already suggested visualizing multi-entries with just a number (here), and it seems to be planned, so both features could potentially coexist.
@timo mentioned some problems that were holding him back from implementing some kind of tag-based coloring, but @GrzegorzCiepka 's solution tackles 3 of the 4 problems. All except:
timo currently tags aren't retrieved from the database when displaying the calendar. When adding this feature, they would have to be queried, which will slightly decrease performance
Honestly, I think this is an inevitable drawback that we will have to experience with any kind of tag visualization. And I believe many of us here agree on the fact that it would be worth it. And, as stated by @timo, it would have to be an optional change, so there would always be the option to just disable it.
Still, there may be some problems with @GrzegorzCiepka 's solution. For instance, if the number of tags is quite high, it will occlude the image (although this happens right now with multi-entries). Maybe this is not an issue for most. But then, we have to remember that the app allows multi-entries, so what tags should it display? All? Just the ones from the first entry? Let the user decide? Those are some decisions that need to be made.
I also brainstormed about this myself, and I came to pretty similar solution/designs. I made an edit to show you how this tag visualization idea (+ revamped multi-entry) could look like in calendar view. In this edit, I made 3 different designs: A, B and C. Let me know what you think about them:

As you see, A is very similar to @GrzegorzCiepka 's design, but it uses squares instead, and it coexists with the multi-entry counter. This counter could be in another format of course, like the ones in B or C. And, of course, dots could be used instead of squares.
B and C are basically the same but with a different orientation. The main idea is to visualize the tag colors in a single bar, thus reducing the space it takes and the potential image occlusion. The drawback is that, when there are a lot of tags, their bar "parts" may be too small to easily identify. But, at a quick glance, you can see which ones have a lot of tags without that much clutter. In my old journal, usually, the more tags a post had, the more important it was 🙂
Personally I like B the most, but let me know your thoughts.
Finally, there is the tag order. From what I know, when multiple tags are added to an entry, the app automatically orders them alphabetically. So, when visualizing them, I guess that order would be followed top->bottom in B and left->right in C. However, I think it would be more interesting to let the user decide the order. For instance, respecting the order with which the user adds the tags to the entry. Maybe two different days may get the exact 2 tags, but one might be more important/relevant in the first day than in the second, so the user can decide to put it first. So, when looking at the calendar, the first colors would be the most informative/important. What do you think?