I’ve read an article on Medical News Today which went into a study that found that testosterone levels spike after every 7 days of abstinence (they would return to normal levels after).
Testosterone spikes can cause increased libido which may be the reason why a lot of people relapse on days 7 or 14 when they first start, or on days immediately preceding these spikes. I’ve definitely felt the urges were strongest on every 7th day of my streak.
My advice to avoid relapsing is to be mindful of these potential spikes on each 7th day of your streak, and the days leading up to it. They should dissipate on the day after so you just need to survive those 7th day spikes.