As far as displaying pleasure and happiness during the days of Eid al-Fitr and Eid al- Adha, there is no harm in that as long as it is within the limits of the Shariah. That would include people preparing food, drink and so forth. It has been confirmed that the Prophet (sallallaahu ‘alaihi-wasallam) said,
'' The Days of Tashreeq are days of eating and drinking and remembrance of Allah.” 
The Days of Tashreeq are the three days following the Day of Eid al- Adha in which the people slaughter their sacrifices, eat from that meat and enjoy the bounties Allah has bestowed upon them. Similarly, during Eid al- Fitr there is no harm in showing one’s pleasure and happiness as long as that does not go beyond the limits of the Shariah.
As for the merriment on the night of the 27th Rajab, the night of the 15th Shaban or during the Day of Ashura, such practices have no source for them whatsoever. Indeed, they are prohibited. No Muslim should attend such celebrations if he is invited to such gatherings. The Prophet (sallallaahu ‘alaihi-wasallam) said,
'' Stay away from innovated matters. Verily every innovated matter is heresy. And every heresy is straying [from the Straight Path] .” 
As for the night of 17th Rajab, some people claim that it is the night of the Miraj in which the Prophet (sallallaahu ‘alaihi-wasallam) ascended into heaven to Allah. However, historically speaking, that is not confirmed. Everything that is not confirmed is false. Even if, hypothetically speaking, that was the night of that event, it is not allowed for us to introduce any new practices with respect to celebrations or acts of worship on that day, since such were not done by the Prophet (sallallaahu ‘alaihi-wasallam). Infact, such is not confirmed from the Companions who were the people who followed the sunnah and the Law of the Prophet (sallallaahu ‘alaihi-wasallam) most closely and enthusiastically. How is it permissible for us to invent something new that was not practiced during the time of the Messenger of Allah (sallallaahu ‘alaihi-wasallam) nor during the time of the Companions?
Even for the 15th of Shaban, there is nothing confirmed from the Messenger (sallallaahu ‘alaihi-wasallam) about venerating that day or praying during its night. Some of the Followers  used to spend its night in prayer and remembrance of Allah, and not in eating, celebrating or taking it as a holiday.
As for the day of Ashura, the Prophet (sallallaahu ‘alaihi-wasallam) was asked about fasting on that day and he said,
'' It is expiation for the previous year .”
Therefore, it is not allowed to display any form of celebration or grieving on that day as such a display of celebration or grieving goes against the sunnah. The only thing that is narrated from the Prophet (sallallaahu ‘alaihi-wasallam) is that one should fast that day , although he also ordered that a person should fast the day prior or the day after Ashurah in order to be different from the Jews who simply fast on that day alone.
 Recorded by Muslim .
 Recorded by Abu Dawud and Ahmed. Muslim also has something very similar.
 This is reference to the generation that followed that of the Companions.