...option which allocates budgets across a calendar month according to traffic demands. The monthly budgeting option has only been made available to selected advertisers during a beta testing period.

Most advertisers would be familiar with how daily budgets work, where a set spending limit is applied each day. One of the shortfalls of this approach means that on lower traffic days, you might have some unused budget leftover that could have been used on higher traffic days.

Monthly budgeting eliminates this issue by automatically adjusting your budget each day to match traffic demands. From the AdWords Help Center:

With a monthly budget, we'll adjust your budget automatically each day to meet traffic demand while still respecting your budget for the calendar month. For example, if your ads often receive less traffic on non-business days during a month, we'll apply your unused budget to remaining business days to take traffic fluctuations into account.

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