This is a complicated question. I made it quite simple and wrote as a comment about two personal anecdotes:
In 1987 I was working at the Islamic University of Yogyakarta (then still IAIN) as an international visiting professor. One morning I received a visit from an English young man, a musician who held a similar position at the Arts Academy of Yogyakarta. He told me that he was in love with a nice Sundanese, West-Javanese lady and wanted to marry her. One condition of the family (and his fiancée) was that he should embrace Islam first. He told me that he was born in a religiously indifferent British family, formally member of the Anglican Church. He had read some good books on Islam, learned how to pray and said that he wanted to accept Islam. To me, a non-Muslim specialist in Islamic Studies, he asked advice how to do this. And, when possible, within the time limit of one week! I gave him advice to address the imam of the campus mosque, brought him in contact with this man and a few days later there was a conversion ceremony. I was invited and attended the ceremony. After a speech by the imam, this mosque leader invited this young man to renounce Christianity and repeat the Shahadat as a confession of his new faith. As usual there were some sambutan after the formal conversion. I had seen already in the beginning, that I was also on the list of sambutan.
When I was given the opportunity to give my talk, I first criticized the imam. According to the best of my knowledge, there is no need for someone who once had formally embraced Christianity, albeit in a modest and not really active way, to renounce Christianity as a whole. Christianity is accepted as a religion, sent by God to mankind, preached by Jesus, son of Mary. But, I still praised this young man for his step to take religion serious in his marriage and to become more active religiously. From a sleeping believer he had become a more practising faithful and so I could happily attend this ceremony.
A report of this ceremony was included in the magazine Suara Muhammadiyah and several priests of Kota Baru, the eminent Jesuit library, criticized me for having accepted the invitation to speak at this ‘Islamisation’, but I repeated my argument that I could be happy with someone becoming religious active, also in a Muslim tradition.
There is a nice story in the Gospel of Jesus about a shepherd who misses one sheep and leave 99 unattended to seek that one sheep. And it concluded with the saying: ‘there will be more rejoicing in heaven about one sinner who repents than over 99 who do not need to repent’ (Luke 15:7)
The second anecdote I want to tell is about a much earlier period. As a Ph.D. student I applied for a stay of three months in the pesantren of Gontor, for participant observation of daily life in the school. I was accepted to live there and follow classes. Finally I asked that I also could join the prayers. I told them that I was a Catholic but did like the style of Muslim prayers. I was questioned about the most common rituals, washing and ablutions, the quick style of praying Al-Fatiha (also Catholics say the prayers in the rosary very fast, like Muslim say al-Fatiha at great speed). Then I was asked to read Surat al-Ikhlas and to comment on it. Allahu ahad .. Lam yalid wa lam yulad: I could convince them that the Christian Creed starts with the confession that God is One and that there is no compromise to this statement. Then I was asked about the second sentence of al-Fatiha (ashhadtu an la ilaha illah Allah). I had to confess that many Christians do not really feel love for Muhammad, known little about him and that he has a bad reputation sometimes, but that I personally love the Qur’an and feel in the Qur’an also the religious and social drive of Muhammad as a great inspirator for 1/5 of mankind. That I consider him a gift of God to mankind and therefore happily join the confession that he is a Servant and Prophet of God. Pak Zarkasji accepted my comments and allowed me to join the Muslim prayers. Although he later prayerd that I should become a ‘full Muslim’ and I said inshallah, may God turn me into a good Muslim in the sense of Muslim with a capital as explained by Nurcholis Madjid, as someone who surrenders to God.
So far, some comments about joy at religious festivals, over the boundaries.