"That are important weeks and months"?
No; that's a sentence fragment. "Those are" should be fine, though. "Das" can have a lot of meanings.
"That is" for singular. "These / those are' plural.
I thought nominative adjectives for plurals should end with -en?
According to the notes section for this lesson, you use the -en ending for plurals if there is a definite article (die) or an indefinite negating article (keine). If there is no article, then you do not use -en.