Must be a country's leader (for governments with both, either head of state or head of government is fine, even if ceremonial) as of Dec 1st, local time, or become such by question close. Doesn't need to die "in office".

Edge case example: a president who leaves office the third day of the month but then dies the fifth counts.

Key Links:

Presidents, Prime Ministers, Monarchs, etc. (most of these are valid, including from non-"country" governments, see caveat below)

Partial Death List

Caveat: members of a council that is itself the Head count, but "deputies" "vices" and "assistants" where there is a higher seat do not.