Grammarbook.com gave the following rules that will help us learn how to use “when” and “whenever”.
Rule 1 – If an event is unique or its date or time is known, use when.
The game will begin Friday evening when the clock strikes seven.
Rule 2 – Whenever is best used for repeated events or events whose date or time is uncertain. If you can substitute every time that or at whatever time that in your sentence, then whenever is preferred.
Whenever I get in the shower, the phone rings.
Source: Grammar Book
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