
I assume her birthday is 2nd April, not that that's when she's 6 months old - or else she's not yet born! ;)
The signs you need to watch out for are increased frequency of urination, swelling of her vulva and increased cleaning of that area. When you start to notice any of these, press a clean white tissue against her vulva first thing every morning. As soon as you see a pink stain, count that as Day 1 and keep her away from all male dogs for at least 21 days - 28 to be on the safe side. Don't even let her out in your own garden unattended.
If you can only get your male out of the house for two weeks of the three (or four) of her season, the 2nd and 3rd would be the best.