We could say there was an occasion (or time) when i was drinking tea An example of using occasion to describe a particular event or time when something took place would be "on one occasion I stayed up until two in the morning" "on occasions" is sometimes used instead of "sometimes" eg. On occasions i go fishing Have a look at this link which explains the various uses quite clearly http://www.oxforddictionaries.com/definition/english/occasion

good: It was an occasion I was drinking tea. yes occasion works here but I like "a time" better in this context