copied from http://www.immigration-law.com/Canada.html:
Any I-485 applications which are receipted by the field offices of the USCIS on Tuesday, August 1, 2006 will be subject to the visa cut-off dates in August 1, 2006. A number of family-based and employment-based immigrant categories for some countries will move backward on that date. Since most of the USCIS Service Centers and local district offices do not accept any mails or overnight delivery services during the weekend, today, Friday, is practically the last day to ship out I-485 filings before retrogression via overnight delivery services such that the agencies can physically receive it on Monday, July 31, 2006. Remember that post-mark on July 31, 2006 is not enough and delivery to the P.O. Boxes of the field offices may not guarantee "filing" in some situation. Accordingly, it will be prudent that people use commercial overnight delivery services and street addresses of the government offices. There may be some people who will have to use the "Saturday pickup" by the commercial delivery services with the guaranty of delivery on Monday, July 31, 2006, but there can arise some unanticipated delay of actual delivery of the packet, for instance, mechanical problem of the freight carrier or weather, unless absolutely impossible to ship out within Today, one may not rely on the Saturday pici-up services. There are some Indians or their spouses who arrive at the port of entry in the U.S. late afternoon, today. These people are those who may be forced to utilize the Saturday pick-up services of the commercial delivery services. In the event that the commercial carrier fails to pick up the package at their offices or home during the day, they may want to take it to their stations in the inernational airport or their other central station to assure a truly last minute filing. Some retrogression is very steep, such as EB-2 for India. Spouses and children can also face a forced separation or a crisis for the qualified principal aliens unless their I-485 applications are shipped out today. Remember also that adirect delivery of the filing in person at the Service Centers and local field offices are in most cases not avaialble. Accordingly, for some offices, even if they drive over to the field offices, they may not be able to file their application on Monday, July 31. Please do not count on the potential opportunity to deliver their applications in person. Good luck.