Once again, I'm scheduling an event not necessarily to have a place and time for ppl to meet, but more for a forum for ppl to get in touch with each other make arrangements on their own. In fact, it might rain tomorrow. Sunday should be better, but a lot of times there is a tournament at Rainbow. To find out if there is a tournament, you can go here (Rainbow is 彩虹河濱公園): http://tsh.tms.gov.tw/Rent/Public/place_2_data_Query.aspx
It's been my experience that the best time to go there is after 3, 3:30pm because most ppl go there early afternoon and start finishing up. Weeknights, after 9:00 is good. You can get a good 1 hour and a half for the lights turn off at 10:30pm.