Learn Swedish: Han jobbar det tuffa skiftet varannan vecka. - He works the tough shift every other week.

Varannan comes from combining var (each/every) and annan (other). It is used with en-words like vecka. Notice that the noun following it must be in the indefinite form, which is why it is varannan vecka and not varannan veckan.

What if the time period was an ett-word, like år (year)?

Because the distributive pronoun has to match the gender of the noun, it changes to vartannat for ett-words. To say "every other year", you would use vartannat år.

Why do we say det tuffa skiftet instead of just det tuffa skift?

This is a great reminder of Swedish "double definiteness." When a definite noun has an adjective attached to it, you need the freestanding definite article (det), the definite ending on the adjective (tuffa), AND the definite ending attached to the noun itself (skiftet).

I see tuffa ends in an "a". Is that because it's plural?

Not in this case! An adjective gets an -a ending when it is plural, but also when it is definite. Because it follows the definite article det, the base word tuff (tough) gets the definite -a ending to become tuffa.