There used to exist a word in Dutch, des, which was part of the 'genitive' case -- the genitive case was used, among other things, to indicate time, e.g. des maandags.
The word is now archaic and isn't used in everyday speech, and the 'genitive' case distinction no longer exists in Dutch, but they still use 's as a shortened form of des, to indicate time.
Appels and appelen cannot be used interchangeably. Appelen is not the plural form of appel. You may be confused about this because it is true for potatoes.
Potato= aardappel, plural aardappels or aardappelen. Apple=apple, plural appels.
When in doubt, check our leading dictionary, the Van Dale dictionary: http://vandale.nl/