I hardly ever disagree with Yinepu, but this time I'm going to. I'd lock down on the 18th, around the same time of day that you set them. This is your first hatch so you don't have any history to tell you therwise.
You set them in Saturday the 31st, so they should hatch on Saturday the 21st. Theoretically they should hatch on the same day of the week you set them. That's just theory though. Reality does not always match theory. There are several different things that can affect actual hatch dates. Average incubation temperature is a big one, but there are some others. I've had mine pip three days early with a warm incubator. Some people have had hatches that late due to cool incubators.
I don't think the actual time of lockdown has to be that precise. To me, the important thing is to go into lockdown with the raised humidity before the eggs first pip. Since you don't have that history to know how to adjust yours, I'd stick with the standard of locking down after 18 days of development.
I don't like the way we talk about days. To me Day 1 should be the day we put the eggs in the incubator, but it is not. An egg does not have 24 hours worth of development 2 seconds after if is put in the incubator. It takes 24 hours for the egg to have a day's worth of development. Since you started them on the last day of the month, you made it easy. You should have 18 days of development on the 18th, which is when you should go into lockdown.
Once you get some history with your incubator, you may want to make some adjustments. Mine always hatch a little early so I go into lockdown a bit earlier than the standard.