When does my mother's 'iddah period end following my father's passing on 9/3/2012 at 8:20 AM?

Your mother's 'iddah (waiting period) begins from the moment of her husband's death. If she is pregnant, her 'iddah ends upon the delivery of her entire pregnancy. If she is not pregnant, her 'iddah is four months and ten days. The 'iddah is calculated by lunar months, with the month of death being completed to thirty days if the death occurred during the month. If your father passed away on 16 Rabi' al-Thani 1433 AH, her 'iddah would end on 26 Sha'ban 1433 AH. If there is any doubt about the date of death, it is preferable to be cautious by adding an extra day. The 'iddah ends at the moment of death, but according to the Maliki school, the day of death is not counted if the death occurred after dawn.