No it is NOT forbidden to be happy and have a celebration...
For example:
(Nehemiah
8:9-12)9 And Ne·he·mi´ah, that is, the Tir·sha´tha, and Ez´ra the priest, the copyist, and the Levites who were instructing the people proceeded to say to all the people: "This very day is holy to Jehovah YOUR God. Do not mourn or weep." For all the people were weeping as they were hearing the words of the law. 10 And he went on to say to them: "Go, eat the fatty things and drink the sweet things, and send portions to the one for whom nothing has been prepared; for this day is holy to our Lord, and do not feel hurt, for the joy of Jehovah is YOUR stronghold." 11 And the Levites were ordering all the people to be silent, saying: "Keep quiet! for this day is holy; and do not feel hurt." 12 So all the people went away to eat and drink and to send out portions and to carry on a great rejoicing, for they had understood the words that had been made known to them.Things like that are not necesarrely bad or "valueless" .... a knife can be used good and bad ... (for dinner or killing somebody else) .... if you are happy and have reasons to be ... no problem .... it will only be "valueless" if your are doing things there that Jehovah hates... some celebrations he hates as you surely will know ... like birthdays ....
(Galatians
6:7-9)7 Do not be misled: God is not one to be mocked. For whatever a man is sowing, this he will also reap; 8 because he who is sowing with a view to his flesh will reap corruption from his flesh, but he who is sowing with a view to the spirit will reap everlasting life from the spirit. 9 So let us not give up in doing what is fine, for in due season we shall reap if we do not tire out.