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?