Thailand is very active in Z26 and there are a few guys with huge signals out of HS-land. My experience has shown they're in the loudest around the equinoxes, usually between about 1400-1600z and again 2300-0100z on the East Coast. Late-March, April and May, late September through late October are the best for me in NJ.
My hardest zones were 19, 23, 24, 26 and 27. Zone 23 was the last. With even a small amp and a decent antenna, however, it's easy to work all 40 in under a year (I've done it every year since 2006).
I've recently had XV2RZ to myself, along with several others from zone 26 - they just don't hear me. I know many times the local manmade noise is very high so its difficult for them to pick out weak signals. I need to do some antenna adjustments (not 20 element monobander at 100' of course haha) but something real. It kills me to hear all these guys bouncing around 20 to 10 meters and not be able to get through. Its annoying because its just that 1 zone (also because it would be nice to have WAZ
). Many times they have great sigs and normally if I can hear them I can work them. The North Pole gobbles up my sigs and spits them out.
One of these days - and your right - spring and fall are the best times. Sunspots, don't fail me now!