'When' usually refers to one specific time. 'Whenever' means ' any/every time that..'. It translates into Spanish as 'cuando sea', 'cada vez que' or 'siempre que'. 'When should I come and visit you?' 'You can come whenever you want. I'm always at home'.

From http://www.learnersdictionary.com/definition/when: A. when /ˈwɛn/ adverb 1 : at what time When will you return? 2 a : at, in, or during which It was a time when people didn't have to lock their doors. the happy days when we were together b : at or during which time We're still waiting for the test results, when we'll decide our next move. 3 informal : at a former and usually less successful time I can say I knew you when. [=I knew you before you were famous or successful] B. when /ˈwɛn/ conjunction Learner's definition of WHEN 1 a : at or during the time that We went fishing when [=while] we were on vacation. I'll leave when you do. [=I'll leave at the same time as you] b : just after the time that You can go when the bell rings. Call me when you get home. c : at any or every time that I cry when [=whenever] I hear that song. When he watches television, he falls asleep. 2 a — used to say what happens, is true, or can be done in a particular situation When you have no family, you are really on your own. We take the bus to school when it rains. A contestant is disqualified when she disobeys the rules. b — used to ask why or how something is done in the situation that exists Why buy a newspaper when you can read the news online for free? How can he buy the house when he has no money? [=he has no money, so how can he buy the house?] 3 : in spite of the fact that : although She quit politics when she might have had a great career in it. 4 : the time or occasion at or in which Tomorrow is when we must decide. C. when /ˈwɛn/ pronoun Learner's definition of WHEN : what or which time He retired in 1998, since when he has been devoting his time to gardening. You need the report by when? Whenever is a conjunction, cannot be used for a question. Example: "we went to the movies whenever we felt like it" As a conjunction, when and whenever are interchangeable