I'm going for an exchange programme to Nimes, France from Delhi, India, in August, just in time for La Tomatina in Buñol. I was planning to take the overnight bus from Nimes to Valencia that arrives at 8h00 on the morning of 31st August. Then a bus from Valencia to Buñol that will take around an hour. Roughly, if everything goes as scheduled, I should reach Buñol at 9h15. The festivities start at 10h.
Is this travel schedule feasible?
If you arrive to Valencia at 8:00 in the morning there is a regular line bus from Valencia to Buñol that leaves Valencia at 9:00 and reach Buñol at 9:55 (according to the official schedule), so if you manage to be in Valencia before 9:00 it is possible to be in Buñol in time.
Going from Nimes to Valencia by road, using a bus in your case, is a travel of more than 7 hours, considering that you have no problems on road. August is a very touristic month in Spain and August 31th is an especially conflictive day because an immense amount of people will start working on September 1rst. So if you can, plan your trip with more margin, sleeping in Valencia for example, to give you some more time to avoid schedule problems.
I attended La Tomatina 2 years ago. I'd suggest you take a few things under consideration:
1) There are a TON of people. Campers, drivers (I slept in a car), and busload after busload of drunken Australians (sorry, but there were!). As such, there will be traffic delays, and buses will be jammed.
2) While La Tomatina was fun, in Requena nearby the night before is the Wine and Water festival. You take a glass/vessel and parade around town, yelling 'agua, agua!' and residents open their windows and poor water on the crowd. After a while, a truck pulls up and you clamber onboard and use your glass to scoop red wine out of barrels on the back! And then this repeats for about 3 hours, fantastic fun!
3) Be VERY careful about your posessions. Pick-pocketing is rife - I had velcroed closed pockets and still didn't feel a thing - phone, money and rental car keys, and judging by the line at the police station, it was a big pay day for the thieves!
4) Finally, have fun - and as a top tip - the goggles, they do nothing! And also, at the end walk down the hill - there's a river you can wash yourself off with ;)