When you ship depends on what you said in your listing. You should ship on the first available "business" day according to what you said in your listing. My listings say one day. So if I get an order on Saturday evening, I would ship on Monday (Tuesday if Monday is a holiday).
Businesses, including eBay sellers, consider "business days" as Monday - Friday and exclude holidays. I put a note in all of my listings to this effect, just for people who might not know, although most people do know this. I say in my listings that I do not ship on Saturday and Sunday or holidays. The truth is, if I am available, I do ship on Saturday, I print my own labels and either give the package to my route carrier or take it to the post office (I knock on the inside door like for package pickup) and hand them my package.
Because some people forget about holidays, if there is a holiday that delays shipping, I drop the buyer a note that I will ship on the first day after the holiday. Sometimes if I get a payment on a Friday, and I know I can't ship on Saturday, I will drop them a note, because even though I am shipping on time on the following Monday, to an anxious buyer it seems like a long time.
Here info about Shipping Pointers:
Hope this helps.