"I thought all adjectives of plural nouns have the -en ending" - That's not the case if there is no article word (a, the, my, etc.). Then you have to use the so-called "strong inflection" for the attributive adjective.
"Kalender" is used without an article word (= strong inflection), nominative, and plural. Thus, the attributive adjective has to be strong inflection, nominative, and plural, too.
See this chart:
A definite article is used together with the noun, but in this case it is on its own to the left of the verb so is considered a demonstrative pronoun (e.g. this, that, these, those) and does not affect the inflection of the adjectives on the other side of the verb.
If you consider a slightly different example:
Das sind die persönlichen Kalender = These are the personal calendars
Here, die is the definite article so we need to use weak inflection rather than strong and persönlich takes an -en ending.