You are given the following price history for the December silver futures:

Day	Futures Price	Daily Gain
June 29	18.62	0
June 30	18.69	700
July 1	18.03	-6600
July 2	17.72	-3100
July 6	18.00	2800
July 7	17.70	-3000
July 8	17.60	-1000

On which days did John receive a margin call?