Matronalia, the Roman equivalent of Mothers' Day.
On the hill that now has the name of Esquiline, A temple was founded, as I recall, on this day, By the Roman women in honour of Iuno. But why do I linger, and burden your thoughts with reasons? The answer you seek is plainly before your eyes. My mother, Iuno, loves brides: crowds of mothers worship me: Such a virtuous reason above all befits her and me.' Bring the goddess flowers: the goddess loves flowering plants: Garland your heads with fresh flowers, and say: 'You, Lucina, have given us the light of life': and say: You hear the prayer of women in childbirth. But let her who is with child, free her hair in prayer, So the goddess may gently free her womb. - Ovid, Fasti III
It was held on the first day of the (pre-Iulian calendar) year, which was the kalends of Martias and special attention was paid to Iuno as Iuno Lucina - "Iuno the Lightbringer", who was held to watch over childbirth and mothers. The epithet "Lucina" may refer to the light (lux) of childbirth or to the grove (lucus) where the temple of Iuno was built in c. 375 BC on the Esquiline Hill.
In the Fasti, Ovid asks Mars why He would let a festival like this occur on the kalends of His sacred month, Him being the god of war and sort of a guys' god and all; Mars replies that it is in honor of the Sabine women whom his son Romulus stole in order to give Roman men the wives necessary to expand the population of the new city.
"Rome was little," Mars says, and then when Romulus wants to pray about it, Mars tells him that prayers are nice but he'd be better off arming himself and the men of Rome and taking what they needed - women - by force. What other kind of advice would you expect from the god of war? So Romulus and his army looked around and lo! there were a bunch of extra Sabine women just hanging around doing nothing!
So Romulus invites all the neighbors over to take a look at his cool new city, and in the middle of the party gives his men the signal and they run off - where to, exactly, is never specified - with the extra women.
After the "rape" - or abduction, more properly, the Latin word raptio meaning "kidnapping" - of the Sabine women, Romulus goes around to each of them and points out that being a Roman would really be a pretty good gig so how's about it?
Some time later, an army of (justifiably) angry Sabines showed up to rescue them. But apparently they'd waited just a little bit too long; the women now had children, Roman children, and they stood between their Roman husbands and the Sabine army, holding up and waving their children around as proof that they weren't really that much in need of rescuing, actually, but thanks and would you like a cup of coffee for the march home?
The Sabines all took this in good spirits and they and the Romans became allies and everybody was happy.