Curiously, English speakers use both relative adverbs. We don't use them in the same proportions, however. The relative adverb "when" is by far the more common of the two.
A Google search reveals 43,600 examples of "the days WHERE," but a search for "the days WHEN" turns up a whopping 1,310,000! Here are a few examples of each:
"THE DAYS WHERE"
"” Long gone are the days where you have multigenerational families living in the same town and usually under the same roof.
http://www.africana.com/columns/brown/ls20041116insure.asp "” On the days where a specific neighboring town is honored, all schoolchildren in that town receive free admission to the Fair after 2 pm on their special day.
http://www.thebige.com/detpages/bige10034.html "THE DAYS WHEN"
"” The contest is held on the days when the surf is biggest and best as determined by the contest director. Daily start time 8 am.
http://www.triplecrownofsurfing.com/when_is_it.php "” This commercial is a glimpse back to the days when a woman could get everything she needed to maintain a happy marriage down at the supermarket.
http://www.tvparty.com/vaultcomsp.html "” That takes us back to the days when a person's cave was his castle and the largest
corporation was the one that went out hunting for mammoth.
http://www.fortune.com/fortune/bing "” In the days when its monopoly was secure, De Beers regularly bought up any supply of rough diamonds that appeared on the market.
http://www.economist.com/printedition/ PrinterFriendly.cfm?Story_ID=2921462
It seems evident that the more logical relative adverb "when" is to be preferred after an expression of time, even though "where" is also used.
Marilyn