When I was in the market for a G35 / G37 I charted the cost per month over time for several months. I assumed the purchase price was constant (I had a VPP claim code so it was for me). Each month IFS would play with the residual and MF so you would have a wide variety of payments for the same car - and that was at a fixed price. So there was no "best time" to buy with a captive lease, you had to just track it for a while so you would know a good deal when it hit
Then add in that the dealer would ask more or less for the car each month based on sales and / or factory incentives or other factors and it can be impossible to pick the "best time" to lease.
The 3rd party lease banks make it a bit easier, the residual starts high at the start of the model year and drops every month or two during the model year. The best price, but worst residual could be at the end of the model year, best residual and worst price at the start of the model year. The sweet spot may be around Jan-Feb.