I'm waiting for a .com to drop which expired on 4th September and I've worked it out to be 20th November as follows:
Domains names that are not renewed within 40 days of their expiry date can now be restored during the Redemption Period that starts 45 days following the expiry date and ends 75 days following the expiry date.
Therefore I guess the answer is about 8/9th December however I have read some reports whereby, for whatever reason, the names sometimes are dropped sooner. I think it seems to depend on the original registrar.
Rather than back ordering it I'm just checking it every day, I do not need it that desperately however at my normal buying price I'll have it.
Many thanks HuskyPup, very kind of you to work it out for me :-) As for back ordering, it's not essential to get the domain at this moment so maybe check each day nearer to the time as you are, just hope there aren't any DC's tracking it also!