When do new tax years begin and end?
Although many other countries (including the USA) start their tax year dates on 1 January and end it on 31 December, in the UK the new tax year begins on 6 April and ends on 5 April a year later.
So, 6 April 2019 was the first day of the 2019/20 tax year, while 6 April 2018 was the first day of the 2018/19 tax year (which ended on 5 April 2019).
Changes to new tax rules are often introduced at the beginning of the new tax year (ie 6 April).