At the end of Before Sunrise, Jesse and Celine decide to meet again on the same platform at a specific time in the future. Before Sunset and Before Midnight continue their story in the future. But was this how it was planned back when the first instalment's script was written? Did Richard Linklater plan the nine-year gaps? Or was the date decided due more to circumstance than to any deliberate plan?
It was definitely not a plan from the start to have sequels with nine-year gaps. I'm having trouble finding the interview I read where they initially said the first sequel was not planned at the time, but grew organically as they kept in touch. For Before Midnight, Ethan Hawke said in Entertainment Weekly (May 10, 2013 issue, page 45):
... But we never had a serious conversation until Rick and Julie were both in New York in 2010. We sat down at dinner and had a long conversation about it. And we made plans to meet again in six months and we'd all think about it and talk about what's on our minds. And then around Christmas in 2011, Rick and I came to visit Julie in L.A., and that's where we batted out the outline: "What if we had twins?"
Later in the interview, page 46:
After the second one, I would have been surprised if we didn't do a third. But now I don't know. My bet is it's going to be some sort of life project. Like a fictional version of the Seven Up! films. I'm open to it.
That wording seems to imply the first sequel was not planned right after the first movie, and nor was the second sequel planned right after the first. It wasn't unexpected but there was no grand plan in place at the time.
Keeping in mind there are semi-autobiographical elements in the films about growing older, and they are co-written by all three, it's difficult to plan or plot a sequel nine years in advance.
I wouldn't be surprised if the idea for the first sequel really took off in 2001 on the set of Waking Life which had a little vignette featuring Hawke and Delpi reprising their roles from Before Sunrise (1995).
EDIT: From an interview with Julie Delpi for Before Sunrise:
Did you watch the first film to get back into character?
We knew the film pretty well. A lot of things had slipped our minds but we started writing the script in 2002 together.
So the year after Waking Life they started writing the script. When asked (in the same interview) if there would be a sequel in another nine years she replied:
Well, we don’t want to do a sequel in nine years because we don’t want to have this mapped out pattern. If we feel like we really have something to say and people really want to hear something else, and explore and see more and go to other fields because we’d do something very different again, we will do it. But we’re not sure we’ll even do anything.