"They run in April."
Translation:Ritheann siad i mí Aibreáin.
Aibreáin is the genitive of Aibreán. As mí is a noun, you use the genitive to say i mí Aibreáin. Without mí, you don't use the genitive. Without mí, you would also change i to in, as Aibreán starts with a vowel, which would give you in Aibreán, but as explained elsewhere on this page, when used with i, Aibreán takes a definite article - san Aibreán (though I have no idea if that's on the list of acceptable answers either).
i mí Aibreáin or san Aibreán.
Section 1.1.5 (f) of An Caighdeán Oifigiúil says:
Úsáidtear an t-alt leis na míonna Márta, Bealtaine, Meitheamh, Samhain agus Nollaig. Ní úsáidtear an t-alt leis na míonna eile, ach amháin sa leagan calctha san Aibreán.
Is minic a bhíonn mí Eanáir níos fuaire ná mí na Nollag
Tá an Cháisc i mí an Mhárta seachas mí Aibreáin i mbliana
The article is used with the months March, May, June, November and December. The article is not used with the other months, except in the petrified form san Aibreán.
January is often colder than December
Easter is in March rather than April this year
A weird thing I noticed recently when i would speak what i read is that eclipsis and lenition are necessary in speech to maintain flow (course), but when certain words come up they just "feel" like they need to change. Its interesting how somehow, it starts to come on its own.
Lenition has its moments where i mistake it for eclipsis (standalone words, mostly), but in the context of a sentence it would feel unnatural to do one thing over another.
It's not just a simple spelling error. You must use the genitive form of the noun after Mí, and the only difference between the nominative Aibreán and the genitive Aibreáin is that i, so when you leave out that i you are making a grammatical error, not a spelling error.