The Vietnamese word for day is ngày.
The very good news about learning how to say day of the week in Vietnamese is that, assuming you have gone through the lesson on how to say Vietnamese numbers, you have already known it!
As a recap, we have learned in that lesson on numbers that to say an ordinal number (1st, second, third, etc.), we simply add the word thứ in front of the number, with the exception of 1st being: thứ nhất.
Now, to say a day of the week in Vietnamese, we also use the same word thứ in front of a number!