"Osclaímid na litreacha gach maidin."

Translation: We open the letters every morning.

[JeffFoster14]

Then we read them, seal them up again, and send them on to their intended recipients.

[FauxShizzle]

"We open the letters each morning"?

[scilling]

“Each” would be an appropriate translation if there were reason to consider all of those mornings individually.

[FauxShizzle]

How would I say "each morning"?

[scilling]

Gach aon maidin would unambiguously be “each morning”; gach maidin could be either “every morning” or “each morning”, depending upon context.