Csdrhrt

"Annus mirabilis" is an expression which refers to a wonderful year like 1905 for A. Einstein and modern physics.

What would be the equivalent for a single day? Is "dies mirabilis" the correct form?

Answers for weeks, months or seasons are also welcomed. :)

Yes, dies mirabilis is perfectly valid!

You can use the adjective mirabilis with any noun.
You have to use the correct form, but that is fortunately easy.
In masculine and feminine it's mirabilis, in neuter it's mirabile.
Some words are plural (e.g. Kalendae, the first day of a month), and those require plural forms mirabiles/mirabilia.
If you find a word in a proper Latin dictionary, its gender will be mentioned.

When it comes to time, most words (annus, dies, mensis, hora) are masculine or feminine and therefore mirabilis goes as such.
The word "time" itself (tempus) is neuter, you would get tempus mirabile.
Summer, autumn, and winter (aestas, autumnus, hiems) are feminine or masculine, but spring (ver) is neuter.

Not only it is correct, but it is actually attested in several places. For instance, in the Catechesis Celtica (allegedly from the 10th century), we read in relation to Easter Sunday:

Facta sunt in hac die multa beneficia, quia in hac die resurrexit Christus, Filius Dei uiui post uastationem infenri, et solutionem humani generis de ore diaboliet de peccato Adae. Et in hac die debemus laetari, quia dies mirabilis est, dies uenerabilis, dies solemnis, dies lucis et iustitiae, dies principalis, dies salutis humani generis, dies laudabilis, dies magnae gloriae, dies ressurectionis Domini nostri Iesu Christi.

Another mediaeval example comes from a poem by Friedrich Taubmann, published in 1604.

In fact, the use of dies mirabilis to signal an important day seems to be a "common use". For instance, Queen Victoria (re)opened the Royal Exchange (a landmark building of the City of London) on 28 October 1844. A poem dedicated to the event refers to the day as dies mirabilis. There are other examples of this use (e.g. here and here).

Charles Lutwidge Dodgson (aka Lewis Carroll) wrote after a rowing trip around Oxford with little Alice and her sisters (trips upon which it is alleged Charles told the girls the stories later compiled in the "Alice in Wonderland" book):

We went down the island, and made a kind of picnic there, taking biscuits with us, and buying gingerbeer and lemonade ... Considering the wild spirits of the children, we got home without accident, having attracted by our remarkable crew a good deal of attention from almost everyone we met. [...] Mark this day, annalist, not only with a white stone, but as altogether dies mirabilis."

Hora mirabilis is also used in several places. For instance, in this XVII century commentary of the Gospel of John.