When there was a big reform of SDLT in December 2014, the measures took effect at midnight on the day of announcement because we were worried about the consequences of any delay on the market. https://twitter.com/HelenMiller_IFS/status/1280078256952221697
This created a bit of a Parliamentary problem because we needed to pass an SI on the day after the Autumn Statement but we couldn't announce that there'd be an SI on the floor of the House until after the Autumn Statement (announcing the SDLT reforms) had been delivered.
It was my job to go and see Speaker Bercow in advance of the Autumn Statement to say that we needed Parliamentary time on Thursday to pass an SI on a subject matter that I couldn't tell him about and that we couldn't tell anyone that we were even doing an SI until Wednesday.
The Speaker said it was 'highly irregular' but was very gracious when I assured him that it was all for good market reasons and there was nothing underhand about it (e.g. we weren't rushing through something unpopular with no notice on a quiet Thursday).
So if the Chancellor is going to announce an immediate cut in SDLT on Wednesday, there should be a Treasury minister going to see @LindayHoyle_MP shortly to say "we're going to announce on Wednesday an SI for Thursday, I can't say more than that but it's all above board".