text: You really should have started trying a few days ago. The best time would be about 2 days before ovulation to give the sperm time to reach the egg. Sperm are not very fast and could use the extra time especially since the egg has limited viability (24-48 hours). If you don't get pregnant this month, you may want to test next month with a predictor that gives you a day or two advanced notice. If I find the exact speed for sperm I'll modify my answer to give you a more exact idea. Good luck.