The dates may be different for different types of plants/germplasm. What was shown above is for apples. It is on the 1st page of the apple catalog. You can find it at this link.
http://www.ars.usda.gov/Main/docs.htm?docid=10013 and I attach the pdf copy. Apple, cherries, grape come from Geneva, NY.
Pears and a bunch of other fruits/nuts come from Corvallis, Oregon. Their page said Feb 1 was the deadline to order scionwood and dormant cuttings from their collection (
http://www.ars.usda.gov/main/site_main.htm?modecode=20-72-15-00).
You'll have to do some digging for other things at other germplasm sites. For other types of plants, I would recommend you find the repository that handles it and call/email them to find out exact dates for those. Here is a link to all of the repository homepages:
http://www.ars-grin.gov/npgs/rephomepgs.html