In the gardens is a huge conservatory (300 ft long!), built in 1813, which is home to a historic collection of camellias.
Each spring (when the camellias are blooming) there's a Camellia Festival, when as well as viewing the camellias and learning about the history of the collection you can also purchase some of the varieties for your own garden.
I didn't really have an opinion about camellias one way or the other before visiting the Festival, but oh my goodness they are so beautiful. Prepare to swoon:
Utterly enchanting - and a wonderful, cheerful burst of colour in the early, still chilly spring.
This year's Camellia Festival has now finished but the camellias are still blooming - do go along for a visit if you can (the gardens and conservatory are free to visit, hurrah), or make a note in your diary to check out next year's Festival. I highly recommend it!
P.S. You can read more about my visit to Chiswick House & Gardens here.