This sounds exactly like synesthesia[0]. My synesthesia is mostly grapheme-color synesthesia, where I "see" words, letters, and numbers as certain colors in my mind's eye. For me, the letter 'A' is red, which is apparently very common among synesthetes. January is dark purple and March is blue. Not 100% sure if this is part of my synesthesia, but sometimes it seems to manifest in other ways, like how I relate certain songs to animals. The author's synesthesia is extremely interesting though!

On a related note, I wouldn't be surprised if synesthesia is actually more common than we currently think because a lot of people who have it think it's normal or never thought anything of it.

[0] https://en.wikipedia.org/wiki/Synesthesia