Synesthesia has always fascinated me. Granted, I didn't know there actually was a word for it until a year or two ago, but the concept is something I've been familiar with ever since I first started learning numbers and letters. The interest has been rekindled lately by discovering a YA novel on the topic ("A Mango-Shaped Space" by Wendy Mass). Apparently it's genetic and more typical in women (check) and left-handed people (check).
Synesthesia is the scientific term for "mixing up" senses - sounds and feelings have colour, letters and numbers have personality, colours have texture. To those who don't experience it themselves, it sounds absolutely crazy, but to those who do, it's as real as any other colour, sound or feeling.
Most of these are foreign to me, but when I first learned my numbers I was convinced that they had personalities. In general even numbers were bad and odd numbers were nice. I never could figure out 4 though, because it was even but behaved odd.
I don't actually see colours when reading numbers, but definitely associate numbers with colours on a subconscious level. All odd numbers are black, 0 is blue, 2 is yellow, 4 is a very vivid green, 6 is red and 8 is orange. All numbers larger than 9 are then combinations of these.
I see spatial calendars. I think this is very much influenced by the way calendars are actually portrayed everywhere, so I don't know if this is part of the synesthesia or just habit. When thinking of the days in a year I see a big calendar with a column for each month and a row for each day. The columns stretch out to infinity with January of the following year being to the right of December of the current year. The further away from "now" they are, the smaller they get, until they're too tiny to be seen. The weekends are different visually from the days of the week, but whether they're a different colour or just smaller, I can't quite describe. When thinking of the days in a week I see a typical time-table with a column for each day and a row for each hour. Again the columns stretch to infinity with one week following the other, and again the weekends are different visually.
I can write with both hands at the same time. Only the same words, but I can mirror one at will. The most natural for writing with both hands is to write 'correctly' with my right hand and mirrored with my left.
Some letters have personalities ('M' is friendly, obviously, 'B' is a bully etc.), but not all, and none of them have colours. I definitely don't see colours and shapes in the air when hearing sounds, nor do I taste colours or see word-pictures like some other synesthetes do. So if what I have is synesthesia, it's obviously a very mild 'case' of it. I can only imagine how it must be for 'real' synesthetes - it must be fascinating... and probably also slightly distracting at times.