What is "Pentecost"?
You may or may not have noticed, but today is the traditional observation of the church holiday of Pentecost. Unlike Christmas or Easter or even Palm Sunday, it is not often noted in Protestant churches (except Pentecostal ones) but it is an important day both for Ancient Israel and for the Church.
Pentecost is literally Greek for "the Fiftieth Day" on the Hebrew calendar and is set aside in scripture as a day to remember the giving of the law and thank God for His gift of the first fruits of the harvest. It's called "The Feast of Weeks" or Shavuot. (Exodus 34:22)
But Pentecost is also the day when the Holy Spirit came upon the apostles, visible as tongues of fire. They spoke in other languages they did not previously know, and from that time on did miracles through the power of the Holy Spirit (Acts 2:1-31).
There are many church traditions for Pentecost (none of which are scriptural, but they mark how various churches have regarded the holiday in times past). Many churches decorate themselves with the color red, in honor of the Holy Spirit. Some do special teaching or scripture reading in foreign languages, to commemorate the speaking in tongues. And many place a symbol of a dove prominently to remind Congregations to think of the Holy Spirit. The time between Ascension Sunday and Pentecost is often also marked with Fasting and Praying together, marking the time that the disciples waited patiently for the promised coming of the Holy Spirit.