Mak, based on what you're describing, kemungkinan besar 4 Feb was your peak day since the line was darker then, then today (5 Feb) lighter already means you're past the surge, OPK lines get darker leading UP to ovulation, then fade after. The thing is, timing for baby-making is actually a window lah, not just one day, a few of us tried and found that even if you miss the exact peak, the couple days before (when line is getting darker) usually sudah cukup fertile so jangan stress sangat, boleh try a few more days lagi just to be safe!