Sending business greeting cards, as mentioned earlier, is not limited to the holidays alone. You can always send a greeting card as long as you feel like doing so.
However, most of the common practices that businesses do, is to send business greeting cards during Christmas, New Year, and Australia Day. It is always up to you to decide.
Electronic greeting cards can be sent three to five days earlier. With so many emails a business has to deal with daily, you are not certain how early or how late they would receive your greetings.
For traditional greetings cards sent through a mail service, it would be best to send them 2-3 weeks ahead of the occasion, especially for celebrations like Christmas and New Year. Businesses and clients are usually loaded with gifts and cards during these events so it is a smart idea to send one ahead to avoid the holiday rush.