Why are people sharing pics of Israeli flag with 'Dees Day' tag?

People are sharing pictures of Israeli flag and 'Dees Day' tag after two British-Israeli sisters and their mother, Maia, Rina and Lucy Dee, respectively, were killed by suspected Palestinian gunmen in West Bank on Friday. Rabbi Leo Dee, Lucy's husband and the girls' father, requested everyone to share pictures of the Israeli flag in their memory.