Not all expenses are paid monthly. Some come quarterly, semi-annually, or annually. Some don't have any particular schedule to them at all, but are out there somewhere beyond the horizon waiting to bite you on the posterior when you least expect them and/or least have the resources to handle them.

A successful financial plan should include a sub-plan for handling these irregular expenses. NOT having a plan to handle them is bound to lead to having other parts of the financial plan knocked...