The Phra Arthit park has been the longest meet up, and you should find people there everyday of the week!
Saturday is the main day for this meet up from about 4:30 on-wards. This not an official meet up but more of a place where Jugglers and Spinners go to practice, so it is possible that you might find nobody there (especially if it’s raining). On the other hand you might find a large number of people playing, juggling and spinning.
Below is a map of where the park is