What inspires your costumes? Do you base them on a series or fandom, or are they completely your own creation?
I've always liked personifying things - places, structures, landforms, holidays, etc. I've drawn and written characters like this for years. I think there's a lot of "character" in things and concepts we interact with daily, and it's fun to make an actual character out of that. As an extension, it's fun to costume that character, because it's another way to realize the design, it's fun to explain it to people and get them to think about the character of things, and it's fun to think about how to roleplay something that's not entirely human.
Do you give your characters a backstory, or come up with "in-character" explanations for parts of the costume?
Because my characters are based on real things, they come with a backstory rooted in actual history. That's a particularly concrete aspect for things that exist on a human timescale (structures, days, etc), but more abstract and interpretable for things like geologic features.
As for explaining features of the costume, I base them heavily on features of the thing I'm personifying. It has to be recognizable, after all! If the thing is architectural, there's a lot to go off of, design-wise. If it's geological, I tend to base a lot of things on rock types. If it's more conceptual, then yes, all the aspects tend to be related to the history.
(And yes, I've cosplayed as a day of the year, a structure, and an earthquake fault in the past.)
Overall, why do you do it? Do you like the freedom of original characters/designs? Do you like the challenge of coming up with a design from scratch, or do you find it more personal than strict recreation?
It's just fun? I like coming up with the characters, and writing them and drawing them, so why not cosplaying them?
Also, in the case of the San Francisco landmarks I've been cosplaying lately, it's a bit of a way to feel closer to the city even though I can't live there yet.