You should always avoid sending your email marketing campaigns during the holiday time. This is because people are usually on a break and tend to check their emails less often. When they do check, they rush through your email marketing emails and get to more personal & important ones. This means you’ve wasted your email credits and it won’t profit you in anyway.
Worst case scenario, you might have the highest unsubscription rate because people are frustrated to see so many emails from you clogging up their inbox. Especially avoid sending a lot of marketing emails in the months of December, January, July, and August. It doesn’t mean you send your subscribers nothing during this time because sometimes, this could be the only time when you subscribers actually makes an effort to check out your products and services in detail.