This week has THREE days with all multiples of THREE; a pattern is emerging.  See Sunday, Wednesday, and Saturday: 6/21/15, 6/24/15 and 6/27/15.

Today is also a Minus Subtract Day, since 6 – 21 = -15.

And Wednesday is also a half back day.  Go up 18 from 6 to 24 and come back 9 (half way) to 15.