A period of time in the past that was idyllically happy and peaceful. The halcyon is a mythical bird said by ancient writers to breed in a nest floating at sea at the winter solstice, charming the wind and waves into calm.
Oxford dictionary of phrase and fable

Halcyon comes from Greek halkyon, variant of alkyon (kingfisher), from háls (sea, salt) and kyôn (conceiving).
I chose to translate "In those halcyon days when…" with À l’époque heureuse où…