The year running from 6 April one year to 5 April the following year. The tax year 1999/2000 is the year ending on 5 April 2000.
The tax year, or financial year, lasts from the 6th of April through to the following 5th of April. Tax Months work in the same way: from the 6th through to the following 5th .
The 12 months from April 6th of one year to April 5th of the following year.