The other night I watched "Boston Legal" and a couple of the characters went to a synagogue service. The holy day was described as being the Jewish arbor day.
Anyway I was disappointed to see that they had guitar playing during the service!
"Jewish Arbor day" would be 'Tu B'Shevat' (the 15th day of the Hebrew month of Shevat), the new year for the purpose of calculating the age of trees for tithing.
It's celebration is informal, usually eating new fruit (anyone remember 'bukser'?) and raising money for or actually planting trees.
If it does not fall on a Sabbath, I know of no reason why guitar-playing wouldn't be OK.
It fell on Feb. 3/4 this year.