There's no big magic recipe behind these forecast sites and apps. They're all based on exactly the same thing (that's why they'll all say more or less the same) -> moon phase. The basic theory is that every half cycle is good (ie. full/new moon respectively). So every 15 days roughly, you'll get 2 good days with a mediocre day either side. And because the theory behind it is the alignment of sun and moon, it also means that 50% through the cycle (either waining or waxing) is also considered "ok", just not quite as good.
So, all told, you just end up with roughly a 7-8 day good/bad cycle with peaks every 2 weeks and fluctuations depending on the exactly sun/moon alignment at the peak of each phase. Weekend just gone was a 50% moon, my app was telling me to get out there too. I did and I blanked, but that was down to stupidity and not lack of fish
They all just take the same basic ingredient and spice them up a bit differently. I can't help but get taken in by it, but my own running statistics (over almost 12 months) are not very convincing. As yet I've not been able to establish any pattern other than good catches usually following a period of "good preparation and hard work" ;-)
