Go ndéana sé maith duit!
Also, and I hope this doesn't sound too pedantic, when you address someone, you use a special form (the vocative case)--basically, you lenite the first consonant if possible and. if it's a masculine name, you make the last consonant slender if possible. When you write it, you use the particle a:
a Cháit, a Pheig, a Mháire
a Shéamais, a Sheosaimh, a Mhaidhc
When you speak, that little a is kind of ignored because there's no stress on it.
Go n-éirí go geal leat!
I believe it's because those months are named after a holiday: Nollaig = Christmas; Mí na Nollag = month of Christmas = December You may find this blog post and the first comment interesting: https://blogs.transparent.com/irish/ainmneacha-na-mionna-i-ngaeilge-names-of-the-months-in-irish/