The Kalends of each month (i.e. the first day) is sacred to Juno, the Ides (around the middle) to Jupiter. The Nones (in the first quarter of the month) is not sacred to a particular deity as such, though the Nones of a given month may be sacred to this deity or that as much as any other day can.
I’ve long pointed out that a fourth day of each month seems to have had some significance in Gaul, namely the day falling exactly one week before each Kalends. A fourth such day divides the month neatly into roughly equal segments. If there was a name for this day, it has been lost; on the calendars of Grand and the Rhine, it is simply noted, if I recall correctly, as a. d. VIII Kal. (the ‘eighth day before the Kalends’, counting inclusively). Such a schema would fit well with the seven-day week that became popular in Roman Gaul; if the Kalends of the coming month was on a Wednesday, for example, then the last Wednesday of the current month would also be a significant day.
The eighth day before the Kalends was also the one on which the Solstices and Equinoxes were observed. As a result, this day focuses our attention on the rhythms of the solar year, and hence of the seasons and the agricultural cycle that we all know and love. I’ve chosen to keep the eighth day before the Kalends as a day sacred to Rosmerta in my personal practice: Rosmerta, the partner of Mercury’s labours; Rosmerta, bearer of the cornucopia and patera as well as the purse and caduceus; Rosmerta, whom worshippers might invoke using the Latin names of Felicitas or Abundantia.
Surviving depictions of Rosmerta usually show her a bit more stern-looking than the deity I’ve encountered (I proceed here into the realm of UPG). Without detracting from her power or generative dynamism, I see Rosmerta more as a helpful, wizened older female figure, the kind of person who greets you with a disarming wink and a tray of cookies whenever you come calling, but who also has incisive things to say about the way the world is going and what you need to do to improve things.