Chaos Space Marines is on the 8th of September
That said, the starter box will not be being released in September at all.
It's the last Saturday of August along with the White Dwarf previewing CSM. (the 25th).
Updated with More Information to clarify pre-orders and release dates.
The 25th is the white dwarf which is all about the new starter and chaos space marines.
Pre-Orders for the new starter set begin on the 25th, for release on the 1st.
Pre-Order for Chaos Space Marines go up on the 8th and the entirety of their range will be released in two waves on the 15th and the 29th.