wa `Aleykum al-Salam, It's not obligatory but it does have a Shar`ee basis which is good manners and wishing your brothers well and strengthening bonds between Muslims and showing good will and wishing others the best to make them happy ect... ^ All of this has Shar`ee basis. And there is this: كان رسول الله صلى الله عليه وسلم إذا دخل رجب قال اللهم بارك لنا في رجب و شعبان و بلغنا رمضان Anas narrated: When Rajab came the Prophet (SAWS) used to say: "O Allah bless this Rajab and Sha`ban and let us receive Ramadan." So asking Allah to bless the month for you and your brothers isn't odd.