Do not choose solely from the translation when; first identify what kind of event you mean.
Da for a specific past event
Da is the basic choice for a specific past event or period. It does not have to be one instant; it may refer to childhood. The point is that you mean that particular past time.
Da jeg var barn, bodde jeg i en liten by.
When I was a child, I lived in a small town.
Når for recurrence and future situations
Når is used for habitual or recurring situations and also future contexts. A habit may be in the past, so past tense alone does not force da. Ask whether you mean one particular occasion or whenever something happens.
Når jeg kommer hjem, lager jeg middag.
When I get home, I make dinner; context may make it habitual or future.
Check the main clause too
When the time clause comes first, the following main clause begins with the finite verb before the subject. Use a comma after the opening subordinate clause. Choosing the right subordinator does not fix the rest of the sentence automatically.
Da jeg kom hjem i går, ringte jeg en venn.
Ringte jeg, not jeg ringte, after the opening clause.
Try it yourself
Choose a subordinator for a childhood sentence and another for a daily morning habit. Explain the contrast.