From June 18th and then every 4th Sunday of each month after that GSP will be on test from 4pm EDT (1pm PDT/9pm BST/10pm CEST)to midnight (9pm PDT/5pm BST/6pm CEST) or untill participation drops to nothing.
That means that from June 18th every 4th sunday of each month will be party sunday. Where you can show of your leet dancing skills and maybe grab a few bug points or leewt or if it is an extra special ocation maybe something else.