There are 53 Fridays in the 2018/19 tax year, rather than 52. If you are paid on Friday 5th April and will earn more than £11,850 in 2018/19, then you are likely to encounter a ‘week 53’ issue.
A similar issue may also affect you if you are due to be paid your salary or pension on 6th April but will receive it on Friday 5th April instead because 6th April is a Saturday.