1. I noticed that we use “Jeden” for “Jeden Tag”, “Jeden Monat” and “Jeden Dienstag” but not for Woche (i.e. Jede Woche). I assume this is because Tag, Monat and Dienstag are masculine words and in the accusative form here. If this is correct, does the use of “always” always trigger the accusative case?
 
2. I noticed that we use “Letzen” for “Letzen Monat” and “Letzten Dienstag” but not for Jahr (Letze Jahr). Is it the same here as for “Jeden”?

1. Expressions of time without a preposition are in the accusative.
from Google books:
https://tinyurl.com/y4vmmy4l

2 The same, but it should be letztes Jahr as Jahr is neuter.

More explanation here:
https://www.youtube.com/watch?v=lD477SCsCzk