I've booked a few times in Y and when I booked the Sale fare in one of the lowest classes of Y I couldn't book the seat, but one was allocated to me after payment. I was really happy with it, but then because of a connecting flight time shift, they had to rebook me and I got not as good ones. Still ok.

At 48hr before I was able to change to a select few other seats if I wanted to.

The more expensive Y class flights where I've been able to book straight away have been 2 weeks and 6 months in advance. With the later it's been fun seeing if anyone is being slotted in next to me and deciding whether to move to an empty row in the vain hope that no one else sits there! I'm sure the flights will be full where ever I try to sit!