Budget purdah, in the United Kingdom, is the period after plans have been prepared but before the Chancellor of the Exchequer's annual budget is announced, when they refrain from discussing any matters which have relevance to the forthcoming budget.
Chancellor of the Exchequer Hugh Dalton resigned after "ma[king] an off-the-cuff remark to a journalist, telling him of some of the tax changes" in the autumn 1947 budget speech
These days there is no such thing as budget purdah, merely budget news management and budget spin. Parts of the statement are selectively trailed, sometimes in the form of leaks to government-supporting newspapers, but more usually through press releases, in the days leading up to the statement. For all that when I was a Corporate Development Executive I used to have to work late into the night on Budget day working out what the bombs that had been dropped meant for the construction, housing and aggregates industries, government by press release is not an improvement.
(If your memory stretches back further than the 24 hour news cycle you may recall that we have already had a budget in March this year. Don't even get me started on that.)