A letter in a dream often carries something you haven't said out loud -- an unspoken truth, an apology, or news you're waiting to receive. An unopened or lost letter can reflect communication left unresolved, something still waiting for the right moment to finally be spoken.
In the world many evils will happen, when in one month there shall happen an eclipse of both luminaries; chiefly in those places subject to the sign in which they happen. - William Lilly (1602.-1681.)