The reason why PM Lee chose 11 Sept and not 12 Sept is because:
1. Usually on the last day of the 7th month, many devouts and believers will be very busy with the burning of hell notes to the deceased, where got time to help out?
2. As counting of votes is usually done way past midnight, it means that on 11 Sept 2015, LKY would be able to witness the whole proceedings before going back. If on 12 Sept 2015, he would not had known the results.
http://www.chinesenewyearday.com/ChineseGhostMonth.htm
Chinese Ghost Month is from August 14, 2015 to September 12, 2015