What is the ruling on saying "we will celebrate Eid" upon completing the fasting of six days of Shawwal, and is asking "when will you celebrate Eid?" considered a derogation of the religion?

The phrase "second Eid al-Fitr" is not an act of mockery that expels one from the faith. The speaker intends to convey that they will be breaking their fast on the days following Ramadan. However, this phrasing should not be used, as the true Eid is the day of Fitr that immediately follows Ramadan. Ibn Hajar al-Makki al-Shafi'i was asked about the prohibition of calling the eighth of Shawwal an Eid. He replied that this is not correct unless one believes it to be an Eid established by the Lawgiver. As for merely calling it that or displaying the symbols of Eid during it, it is not forbidden, but it is contrary to what is most preferable.