Is it “en” or “à” + cette occasion-là?

Question

If we are to translate "on that occasion", would the choice of en or à dépend on whether we used là at the end or not, i.e. out of the four possiblities

• En/À cette occasion-là
• En/À cette occasion

Which is more natural or correct to you? Does the choice depend on where you are from?

Answer 1

à is the right one. You can use also 'en' but would sound more pompous, like delivering a public speech en l'occasion of the Queen's Platinum Jubilee.