I think an every 2 weeks email is fine- by far!
I would absolutely make sure that you had something to offer within the email- even if it is just a relevant article or conversation, blog post (link to it).... anything... it doesn't have to be a new item or sale item, but you can link to current products you have within your relative content. Or below it, etc.
As for day/night, that depends probably more on what you find most successful. Look at your analytics and see what days/time of day you have the most traffic to your site... maybe send them out ahead of that time, and see if you can drive people there on the lower days...
Anyway, that's just a couple thoughts of mine, I look forward to seeing what other people have to say.
