AKA: Feriae Sementivae, Sementina dies, Paganalia(in the country)
Sementivae is the Roman festival of sowing. It was held every year, but the date wasn't fixed. The time this festival would occur would be appointed by priests / magistrates. It was usually held in late January.
The festival honored the goddesses Ceres and Tellus (Terra Mater). The goddesses would be invoked for the protection against any damage done to the coming seeds to sprout (ie: bad weather). The goddesses would also be prayed to for good crops, as Ceres governed over agriculture, harvest, growth, etc., and Tellus, being an Earth goddess, gives the plants a place to live. Apparently, the first half of the event would honor Tellus, and a week later, a festive honoring Ceres would occur.
Offerings would be made to Ceres and Tellus, including: prayers, food (such as spelled buns) and a pregnant sow would be sacrificed. Oscilla, small clay discs, would be hung on trees, decorated with various figures, with the purpose of warding off evil and negative influences.
If you choose to modernly celebrate this holiday, ways to do so are fairly easy! Many people make oscilla with playdoh or clay. You can plant seeds and sprouts indoors if it's too cold outside to plant, and you can honor Ceres by baking bread.