According to a custom observed by many Jews of Ashkenazic (Eastern European) descent, key shaped challahs are baked the first shabbos after Passover as a symbol of the desire to bring blessings into the home. After the holiday, and the Jewish people's deliverance from slavery, is the time for new beginnings, so it is only fitting that one would want to grace their home with good things, and what can be better than freshly baked challah?
Now, one doesn't have to actually bake challahs shaped like keys, one can also simply place a key (wrapped in aluminum foil) in the bottom of the pan, or under the braided loaf, but in our house the key shaped challahs are certainly more interesting. I have baked large challah keys, medium sized keys and even small keys, and even whole wheat challah keys, and speaking from experience and the desire for the key to stay intact, I'd say a medium sized key like this one is the way to go.
Ah yes, and those little flower details? Oh so simple, just make a small coin shaped piece of challah, cut 5 slits into the circle and pinch the sections to form petals.
Enjoy, and check out my posts here and here, from past years to learn about this special tradition.