There is no report from the Prophet (peace be upon him) as to suggest that there is any special virtue in performing umrah in the month of Rajab or that it is encouraged.
Prophet sallahu-alaihi-wasalam commanded his ummah to fast on Mondays, (and it can be any Monday of the year) as his birthday was on Monday. He didn’t ask for any (Prophet’s Birthday) celebration, rather an act of worship that would benefit only the worshipper.
He was asked about fasting on Mondays, and he said: “That is the day on which I was born and the day on which I was entrusted with the Mission or when I was first given Revelation.” (Reported by Muslim, al-Nisaa’i and Abu Dawood).
It is believed that the marriage that will make her permissible for her first husband must be a valid marriage. Temporary marriage (Mut’ah marriage) or marriage for the purpose of making her permissible for first husband (Tahleel marriage) are harams and invalid. i.e. one cannot marry a woman temporarily with an intention to divorce her and make her permissible for her first husband is haram and invalid. Such marriages don’t make a woman permissible for her first husband. Majority of scholars are of same belief and say so.