The secondary transfer window closes at 10 PM PT. While rosters in MLS do not freeze till Aug. 30, MLS teams signing players under contract, need an ITC transferred, or are on another MLS team currently need to be moved before the window closes tonight. Teams may also sign players via a HGP contract until the roster freeze.
Teams do not have to announce the signing prior to the deadline closing, just have to submit the paperwork to the league. With that being said, most signings and trades will likely be announced or leaked before the window closes.