The 25th is usually the last chance to talk to or visit (or lurk at the end of a school day at) a state school before the CAF submission is due on 31st. So whilst it isn't the deadline as such, many parents treat it as such as there is no one to talk to the week after!
As about 90% of parents do application online the 31st is the deadline, but if you want to do paper application and hand it in to your primary for them to pass it on to admissions have to do it before half term.
In reality most open days are over by second week of October so should have worked out the order of your (upto) 6 choices. The first round test results are effectively a pass/fail for decision making in the sence a fail means not worth including on CAF as you wont get in, so drop that school, move everything else up a place and add another.
Remember it may be sensible to include a banker that you can get into as 6th choice (even if you would prefer to go elsewhere) as unfortunatly many don't get offered any of their 6 choices and end up with some undersubscribed school that not enough people chose even if its not close, or you never listed it