In a small bowl, combine the active dry yeast, 2 tablespoons of sugar, and 1/4 cup of warm water. Stir gently and let it sit for about 5-10 minutes until it becomes frothy.
In a large mixing bowl, combine 4 cups of all-purpose flour and salt. Make a well in the center and add the activated yeast mixture, the beaten eggs, the remaining warm water, and the oil.
Mix until a shaggy dough forms. Gradually add the remaining flour, one cup at a time, until the dough pulls away from the sides of the bowl. Turn the dough out onto a floured surface and knead for about 8-10 minutes until smooth and elastic.
Place the dough in a greased bowl, cover it with a kitchen towel, and let it rise in a warm place for about 1-2 hours, or until it has doubled in size.
Once risen, punch down the dough and turn it out onto a floured surface. Divide the dough into three equal pieces. Roll each piece into a long rope, about 12-15 inches long.
Pinch the ends of the three ropes together and braid them, pinching the ends again to seal. Place the braided loaf onto a greased baking sheet.
Cover the braided loaf with a kitchen towel and let it rise again for about 30-45 minutes, or until it has puffed up.
About 15 minutes before baking, preheat your oven to 350 degrees F (175 degrees C).
Brush the top of the loaf with the slightly beaten egg yolk. If desired, sprinkle poppy seeds on top for added flavor and decoration.
Bake the challah bread in the preheated oven for 25-30 minutes, or until golden brown and the bread sounds hollow when tapped on the bottom.
Remove the bread from the oven and let it cool on a wire rack before slicing.