Or another way to sell papers. On a date of certain (good) historical significance, newspapers will stick in a supplement and declare themselves a "souvenir edition", although it's really just a normal paper with coverage of the event taking up a good ten or so pages.
* The marriage of a celebrity or perhaps a royal couple.
* The election and the inauguration of a new leader.
* New year's day, year 2000.
* Important national events like the Olympics.
* Local team wins a national title.
It may sound vaguely silly, but newspapers published on the day of an event of particular historical significance ''are'' mildly collectable. There are a number of companies that will send you a copy of a particular paper for a particular date as a gift.