I think there are a couple of issues here, because all modifiers and all "nouns" are not treated the same.
First, the usual order is (number/determiner)(measure word)(adjective)(noun). If there is a relative clause ending in 的 used as a modifier, it can be placed either before the number slot or before the adjective slot. Therefore you can say either:
In a given phrase, one order or the other may be clearer or allow you to omit or at least not repeat 的. Repeating 的 is often awkward or bad style.
I have also seen 这是简单的一天。 as another example.
Would 这是一简单的天。 be wrong?
The problem with 天 is that it is both a free noun meaning "sky" and a measure word meaning "day." Its meaning is also vaguer than a translation as "day" might indicate. In the quoted example 天 in the meaning of "day" is "fortified" by adding 一 before it and forming a two-syllable word to allow the two syllables to be used as a free noun together in this meaning. The placement of 简单的 is then unremarkable. If you replaced 一天 with 日子, I believe you would say: 一个简单的日子