Answer: Early spring radishes such as the Crimson Giant Variety need to be grown in a very cool, moist, rich, loose soil at cool temperatures. When the soil or the ambiant temperature is too high, they put their energy into growing leaves with the intent of shooting to seed as fast as possible. This is done at the expense of the roots which turn out small, pungent and basically pretty nasty. I suspect this is what happened to your radishes.
Also, the number of days to harvest on the seed packet is pretty much a guide rather than a rule because your home experience will probably vary a bit depending on the growing conditions that year in your garden. Sometimes you can eat the immature veggies such as radishes or lettuce or zucchini squash before they are technically "ready" and sometimes things like melons or tomatoes just aren't ripe enough due to cool or cloudy weather... -- although in this case I don't think extra time would help.
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