On holidays people tend to not be on their computers and don't check their emails as they normally would. Which means that they will tend get there emails when they return, with a lot of other emails that pile together. So pretty much every email other than their VERY important ones tend to get deleted very fast, without even thinking about it. Of course this is mandatory, but the months to send less emails would be December, January, July and August. Will you lose money if you follow this...? It's a possibility since you are sending less emails, however I would recommend to send your more important emails on non-holiday days. This way they will most likely get read by more people on your list.
Much like press releases, you don't want to reach out to people when they are least likely to pay attention. There are a few exceptions to this rule though. Post-holiday sales can often be a good reason to reach out during a holiday period (although ideally you'd reach out before).
not necessarily. It depends on the Economy and weather. If the weather sucks then most people stay at home. If the economy is bad, most people can't afford to go out... So for me I will test, and I will send... so NOT really not to send your offers out. It depends on your market. If it is B2B then of course, but if it is a B2C then not necessary.
I think there are a few to choose from. I just did a search and found this http://wordpress.org/extend/plugins/wordpress-newsletter/ "simple newsletter plugin"