Proof the yeast in the lukewarm water in a large bowl. Add the sugar, salt, butter, eggs, and 5 cups of the flour, a cup at a time. Beat thoroughly with a wooden spoon or with the hands. Gradually add more flour until the dough is very stiff. Turn the dough out on a board sprinkled with flour. Knead until the dough is smooth and elastic, approximately 10 minutes.

Place the dough in a very large buttered bowl, and turn to coat the surface with butter. Cover and let rise in a warm, draft-free place until doubled in bulk, about 1½ to 2 hours. Punch the dough down and divide into six equal parts. Roll each portion into a rope about 1 inch in diameter on a lightly floured board. Braid three ropes together to make two loaves. Place the breads about 6 inches apart on a buttered baking sheet. Cover and let rise in a warm place until almost doubled in bulk. Brush the tops of the loaves with the egg wash and sprinkle with poppy seeds. Bake in a preheated 400° oven for 35 to 45 minutes, or until the loaves sound hollow when tapped with the knuckles. Cool on racks.