the old days
Synonym discussion
Sense: idiom/1
All five phrases refer to time before the present, but they differ in tone and use. 'The old days' is the most neutral among the nostalgic expressions — it can be used fondly, critically, or just descriptively. 'The good old days' is always positive and strongly nostalgic; it assumes the past was better. 'The past' is the broadest and most neutral of all — it covers everything from last week to ancient history. 'The olden days' has a storybook quality and is often used playfully or in children's literature. 'Former times' is the most formal option, suitable for essays and historical writing. For everyday conversation about one's own memories, 'the old days' is the safest and most natural choice.
Etymology
A fixed phrase formed from the common English words 'old' (from Old English 'eald', meaning aged or long-existing) and 'days' (from Old English 'dæg', meaning the period of daylight or a 24-hour unit of time). The combination has been used for centuries as a simple way to contrast the present with the past.