Every Budget is presented in February and lays out the plan for the government’s spending in the financial year that will start in April that year. This relies on a fair bit of estimation. Once about 8-9 months are past, the government is in a much better state to have an accurate idea of how much it is going to spend in the year. So, in December, it submits what is known as a Supplementary Demand for Grants if it needs more money.