Ah ok well according to
this site (which pulls data from people's fishing logs - though I think it only works on JP clients as I couldn't get it to work using an English one) most people have caught it at night as you have mentioned. The day thing coulda been a rare fluke maybe? Here's the direct link to the
fish. That site now has an English version and I found it very helpful to use for all the big fish catching. I hope the guy makes an English version of the log reader so those of us using English interface can contribute our fishing results.