The budget bills are supposed to arrive on the president's desk sometime in September, just prior to the start of the fiscal year, though they have sometimes arrived for his review as late as December or January. The president then has 10 days to sign the bills into law. He may choose to veto any one or a combination of them and accept others. Once enacted, each bill provides budget authority to allow federal agencies to incur obligations; that is, to enter into contracts, employ personnel, submit purchase orders, and so on. Payment of these obligations, called outlays, is usually made via electronic transfers or checks issued by the Treasury Department.
If the president vetoes a bill, it returns to Congress, where it must receive a two-thirds vote in both houses in order to override the veto and become law. If Congress does not succeed in overriding the veto, the bill dies. By the time budget legislation reaches the president, however, all the decisions have usually been made, and a veto is unlikely unless the legislation is part of a much larger political battle between the president and the Congress, diverging grossly from the president's recommendations. (The last time such a situation occurred was in 1995 when President Clinton, a Democrat, vetoed a budget passed by a Republican majority in Congress because of tax revenue provisions that he opposed.)
In the event that the president does veto a bill or that delays somewhere during the budget process have prevented a bill from being signed into law by the start of the new fiscal year on October 1, Congress usually passes a "continuing resolution" to allow the government to continue making payments to employees and programs until the new appropriations bill is passed by Congress and signed by the president.