Yes, Jews have always observed civic and secular holidays. Some synagogues have a national flag on display and many synagogues participate in interfaith observances of holidays such as Thanksgiving and Martin Luther King, Jr. Day.
Mishkan T'filah, the prayerbook of the Reform Movement contains the national hymns of Israel, Canada, and the United States, and this site includes a number of prayers and blessings to commemorate secular holidays.
