Hi Enri, the coffee chat is always on Wednesdays, but sometimes there is a pub chat which is later in the evening, usually starting about 7pm and running until about 10pm. There are no set activities on the weekend but sometimes people organize a group lunch or brunch through the site.
I am new to Buenos Aires, I was looking forward to this, but I guess next time, I won't be able to make it up in the given time. But will try, may be towards 6:30 pm.