Good old days is a cliché in popular culture. It refers to an era considered by the speaker to be better than the era he or she is currently in. It is a form of nostalgic romanticisation.
An early use is by John Henley in The Primitive Liturgy: for the Use of the Oratory, Part 1. Being a form of Morning and Evening Prayer..., 1726
To all sincere Lovers of their Religion, And of their Country ; of Truth, Learning, Charity, and Moderation ; to all honest Admirers of the good old Days of their best and wisest Fore-fathers, this first Part of the Primitive Liturgy Is most humbly dedicated.
This book is reviewed in The Historical Register, volume 11, 1727
In 1727 Daniel Defoe wrote "In the good old days of Trade, which our Fore-fathers plodded on in ...".