I can't tell you a day per se because your O can change. If you must do it that way (instead of using an OPK or fertility monitor) then you'd want to try on July 31 (if you want a 3 day cutoff), Aug 1 (if you want a 2 day cutoff) or Aug 2 (if you don't care about timing and just want a good chance of conception with one attempt. Perhaps obviously, 2 day cutoff will still be much better chance of conception than 3 day so if you want to split the difference, go with Aug 1.

