In terms of overall numbers, there are several reasons why. The two main ones are:
The established national Japfest brand migrated to Silverstone in 2016.
This was partly as a result of the horrendous queues at Combe every year.
These were causing major grief for the local community (as well as attendees).
Japfest also changed its date to April from the traditional May one at Combe.
The combined effect was a noticeable drop in the support for Japday.
JDMCombe has moved to June which might have some effect in the longer term.
However Silverstone's location makes it more convenient for many more people.
The answer to your question is
here (also, see the list on page 1 of this thread).
M16GLD wrote: ↑13/05/17 21:22
What time do members tend to arrive on the day?
Gates open at 09.00 to the public and 07.00 to Clubs/Exhibitors
http://jdmcombe.com/visitor-information/
Suggest you aim to be on the stand by 08.00 (allowing for queuing time).
There is usually a convoy in from a previously agreed location.*
However it open to anyone to do their own thing if they pefer.
* Will post up a suggested RV once I find out which gate we will be using.