The day the Chancellor of the Exchequer announces new taxation proposals and the latest forecast for the economy and public finances.
Not in the UK.
Government spending is authorised through Estimates that occur on a separate cycle. Though the Budget can outline future changes to spending, the primary effect of the Budget is to propose changes to taxation.
The Government proposes a Finance Bill that contains all of the Budget Proposals in the format required to enact them into law. The Bill is then debated and can be amended exactly as other Bills can be, it is very rare (VAT on fuel in 1994 as one example), but not unheard of for Budget proposals to be amended by Parliament after they are proposed.
Special rules exist for bills that relate to Government spending in the House of Lords. For the Finance Bill, the House of Lords holds a general economic debate for Second Reading, but passes the remaining stages without debate.
Definitely not. The process is characterised by secrecy, the Cabinet is only informed of the Budget details on the morning that it is announced. Unlike other Ministerial Statements, the Opposition is given no advance sight of the statement with which to base their reply. The reply to the Budget is given by the Leader of the Opposition rather than the Shadow Chancellor.
On Budget Day the Treasury publishes a 'Budget Report', which is more commonly known as the 'Red Book' from the distinctive cover. During the Budget Speech you will observe the backbenchers rifling though its pages to understand the Budget proposals and any interesting details.
The time of year has been in flux over the last few years, but tends to be either Autumn or Spring. Whichever is used, the opposite season is used to make a 'Spring Statement' or 'Autumn Statement' that may contain further budgeting measures.