When an ad set has a daily_budget, you can expect relatively even spending from day to day while the ad set is live, but there can be some variance between days. On each day, your actual spending could be up to 125% of the average daily budget. However you will not spend more than 7 times the average daily budget over a calendar week. A calendar week is Sunday to Saturday. For example, for an ad set with daily_budget $100.00, you may see spending on a certain day to be as high as $125.00, but weekly spending will not exceed $700.00. If the ad set does not start on Saturday, the first week's spending limit is prorated based on the day and time the ad starts.