"15th of Shevat;" New Year of the Trees; Jewish Arbor Day, which is a minor festival.
Literally, “four species.” The Torah specifies four species to bring together on Sukkot. The four species are: lulav (branches of palm trees), etrog (citron), hadasim (myrtle branches), and aravot (willows) (Leviticus 23:40).
"Counting of the omer;" An omer is a biblical measurement of grain. The counting of the omer is 49-day period that begins on the second night of Passover during which each day is counted with a blessing. On the 50th day, the Festival of Shavuot is observed.