According to popular belief, the custom of afternoon tea began when Anna Maria Stanhope (1783 - 1857) got a bit peckish in the afternoon. Also known as the Duchess of Bedford, Anna Maria was a lady-in-waiting to the young Queen Victoria.
The main meal of the day (for wealthy people at least) was the evening meal. Anna decided that she needed a little sustenance between the normal light lunch and dinner, and started to have her servants serve her sandwiches, cakes and tea. A similar idea had been followed by many less wealthy people for quite a while (probably without the cakes!), But Anna Maria made it acceptable, and even popular, for ladies who were very high-class.
We have Anna, therefore, to thank for popularising the idea of afternoon tea, although she didn’t actually invent it.