Alex, I am an attorney, but not a California attorney, so I'll speak generally. California can be a little quirky at times, so you should definitely contact your closing attorney to help you out. Generally speaking, just reserving a name is not enough. For an LLC to exist, most states require a certificate of formation to be filed with the Secretary of State (or some equivalent government entity). If the LLC is formed in the same state as the property being purchased, then that's typically the only filing you need (along with a fee). If the LLC is formed in a different state, then the LLC would also need to be qualified to do business in the state where the property is located. These fillings can be done very quickly in most states (often same day). Your local attorney should be able to help you out.
The LLC would also need an operating agreement describing how it is managed and how money is shared among members. If you are investing with others through the LLC, then that document would need to be negotiated, agreed upon and executed. If you are the sole member, then this document can also be done very quickly.
Bottom line is that there are steps you'll need to take, but if you use an attorney in your area, they can be done quickly and you have plenty of time if you are scheduled to close on Monday.
Hope this helps.
Jarrod