The where clause is MySQL. MySQL has a function CURDATE() that returns the current date, so you don't need to generate it yourself. Also, 'date' is a reserved sql word, so you'll need to escape it with backticks.
This should work (not tested - but refer to MySQL documentation for further info):
Thanks for the answer, but it does not work. There is still the error "Couldn't run query [...] right syntax to use near '5 WHERE (`date` > CURDATE()) AND (`SiteTree_Live`.ClassName IN ('event')) GROUP' at line 1"