Your packet instructions are telling you to keep the seeds moist after you have sown them, that's standard procedure. Once moisture has breached the seed coat and reached the seed embryo, they can't be allowed to dry clear out again before germination or the seed will die. You'll want to sow the seeds, then keep the sowing medium (whether you have sown in pots or directly in the ground) consistently moist. Not soggy, moist like a squeezed out sponge moist.

What were your plans for these, pots, or directly sown where they are to grow. Do you have the supplies you need if you had planned on pots. The only seeds typically placed in water for a long soak before sowing are large seeds with a very hard seed coat - a soak can help moisture begin to penetrate then but you have none of that type seed.

You may get better germination from the astilbe if you sow those on the surface of a prepared seed pot (some light may be beneficial to germination) and place the pot, moist and wrapped in saran wrap, into your refrigerator for a couple of weeks before bringing back out to warmer. If you are still having frosts, you could accomplish the same thing by putting the pot outdoors and leaving it several days. A period of moist cold (called stratifying) can help to break the dormancy of the seed and prepare it for germination when it is again in warmer temperatures. 35-40F is a good temperature for accomplishing this, refrigerator, not freezer.

Do you use your local library? A really great book with many helpful illustrations that explains the whole phenomena of germinating seed, all the what's and why's, is Making More Plants by Ken Druse. I think you might enjoy it....