Estar: preterite vs imperfect ?

[AlwaysLost]

So my basic understanding of preterite vs imperfect is that emotions stated in the past need to be imperfect. For example:

Estoy triste - I am sad
Estaba triste - I was sad


But are there an exceptions that would allow "Estuve triste"?

[AngelicaDeAlquezar]

When there is a specific time frame for when you were sad; there is a specific moment when the emotion was felt.

- Estuve triste toda la semana porque no me llamabas. -> One week.
I was sad the whole week because you wouldn't call me.

- Estuvimos tristes hasta que el perro regresó a la casa. -> From the moment it disappeared to the moment we found it.
We were sad until the dog came back home.

- ¿Cuántas veces estuviste triste y te hice reír? -> Countable moments
How many times were you sad and I made you laugh.

- Mis amigos estuvieron tristes durante la fiesta porque no había alcohol. -> One occasion
My friends were sad during the party because there was no alcohol.