Q. I planted turnips from six-packs in September. I harvested two beautiful turnips but their flavor was terrible, approaching inedible. They were very bitter. What may have happened?
A. The timing for planting sounds okay but heat will do that to turnips and many other root crops. Turnips have a higher germination threshold temperature, about 60F, and so have to go in a bit earlier in the fall than beets, parsnips or rutabagas and later in the spring. Also, a lack of water would too but if they are a good size and shape then I doubt it is a water problem.
Another thing to note is that turnips, like rutabagas, can contain a bitterness that some people find intolerable. This is an inherited trait in people so that some people will taste the bitterness while others may not. You may be one of these people. Also, you might try boiling them first, throw the water out half way through boiling and replace it. This may help remove bitterness.
I would switch to a different variety or grow kohlrabi instead which I find much more enjoyable and easier to grow. Make sure your soil has been prepared with plenty of compost and phosphorus prior to planting. Make sure they get adequate water and I would even mulch them with some straw after they emerge if the temperatures are still warm.
I got an update. Turnips harvested later were not bitter so sounds like they were planted a bit too early and got hit with some high temperatures.