Q:

Can "diario" and "periódico" be used interchangeably?

I just stumbled upon "diario" and "periódico" while extending my vocabulary. Both mean newspaper in English.
Is there any context in which just one fits? Can they be used interchangeably?

A:

Periódico is a newspaper published periodically, with the period not specified. But also can be used as diario. According to RAE:

3 adj. Dicho de un impreso: Que se publica con determinados intervalos de tiempo. U. m. c. s. m.
  6 m. Publicación que sale diariamente.

Diario is a newspaper published daily. According to RAE:

m. Periódico que se publica todos los días.

Therefore, a diario is a periódico, and both can be used to refer to a newspaper published on a daily basis.
You also have the word semanario to specify a publication published every week.

A:

Periódico means periodic, that is, periodic, regular, recurring.
Diario means daily.

Hence, diario is something more strict that periódico because the first refers to a daily periodic.
However, currently both are used almost interchangeably and so DRAE states:

periódico, ca
Del lat. periodĭcus, y este del gr. περιοδικός periodikós.

adj. Que guarda período determinado.
adj. Que se repite con frecuencia a intervalos determinados.
adj. Dicho de un impreso: Que se publica con determinados intervalos de tiempo. U. m. c. s. m.

...

m. Publicación que sale diariamente.

diario, ria
Del lat. diarium.

adj. Correspondiente a todos los días. Rutina, comida diaria.
m. Relato de lo que ha sucedido día por día. Escribe un diario íntimo. Los diarios de Jovellanos.
m. Libro o cuaderno donde se lleva un diario.
m. Periódico que se publica todos los días.

I cannot think in any periódico that is not diario. You can find semanarios, etc, but nowadays I haven't heard of the use of periódico to refer to something that is not a diario.

