Google Sheet day() formula returns wrong day name

I want to get Day name from a specific date on Google Spreadsheet (also tried on Microsoft Excel Online).

I have written specific date on a cell B1: 2020-03-31

then custom number format it as: yyyy-mm-dd

I also have a formula on it's side in A1 =day(B1) the result is Tuesday. This is correct.

But when I enter a new date: 2020-04-01 the result of the formula have gone wrong.

screenshot of the issue

What's causing this and how to solve it? Did I use the wrong formula?

Thanks.

edit: found the issue it was because column of A/C was formatted as number of day of month, not the date itself.

correct formula would be: =TEXT(B1,"dddd")