Muharram and Safar
Dhul Qa'dah and Dhul Hijjah
Shawwaal and Sha'baan
Rajab and Rabiul Awwal
They are the least important of the pillars. They are the least important pillars of Islaam.
They only occur once a year, whereas the other pillars occur more often.
It is permissible to leave them out of choice.
They are already included in the words "Treat the prohibited as prohibited".
The incident might have occurred before Hajj became obligatory.
The might might have been a poor man who did not have any zakaah to pay.
The question shows the reluctance of the companions to do good deeds.
The question shows the enthusiasm of the companions in asking beneficial questions.
The hadeeth shows the ignorance of the companions that they had to ask questions.
Initiating the greeting of salaam to another Muslim
Praying the obligatory prayers
Giving the zakaah
Treating the lawful as lawful (believing it to be lawful)
Treating the recommended as recommended and doing as much of it as possible
Treating the haraam as haraam (abstaining form it believing it to be haraam)
Praying the sunnah/rawaatib prayers regularly