On days when Tachanun is not said it is the minhag in K’hal Adath Jeshurun to indicate this omission by illuminating lights on special sconces installed for this purpose, located on the mizrach wall. The two sconces, one on either side of the Oron, consist of seven lights which can be lit in combinations of one, two, three, four or seven lights depending on the importance of the day. The greater the importance the more lights.*
The following is the official chart of which non-tachanun days warrant how many lights:
One Light - Bris, Chosson
Two Lights - Chodesh Nissan, Lag Bo’Omer, 2 Sivan – 4 Sivan, 15 Av, 15 Shvat, 12-13 Tishrei, 25-29 Tishrei
Three Lights - Rosh Chodesh, Chanukah, Erev Yom Tov, Shushan Purim, Purim Koton, Shushan Purim Koton, Isru Chag, 11 Tishrei
Four Lights - Chol Hamoed
Seven Lights - Shabbos, Yomim Tovim, Hoshanoh Raboh, Purim
*On those days when three or more lights are lit, Lamnatzeach (between Ashrei and Uvo L’Tziyon) is also not said.