Auspicious date and time for Marriage are calculated using traditional vedic astrology rules.
Generally, marriages are avoided when beneficial planets like Jupiter and Venus are combust by sun.
Few people avoid marriages when Sun transits sagittarius and pisces (nirayana method).
Marriage is also avoided in certain lunar months.
After eliminating all those dates, pick a day when moon transits in these constellations (nakshatras) : Rohini, Mrigasira, Magha, Uttara, Hastha, Swathi, Anuradha, Moola, Uttarashada, Uttarabhadra, Revathi.
These days are considered as best.
When moon transits Aswini, Chitra, Sravanam, Dhanista, it is considered as moderate day for marriage.
Rest should be avoided.
Also in a lunar month, pick only 2,3,5,7,10,11,12,13,15 days.
The day after newmoonday(amavasya) is also considered as good.
These week days should be picked : Monday, Wednesday, Thursday, Friday are best.
Sunday and Saturday are moderate days.
Tuesday should be avoided.
Leaving adhika maasa(extra month added once every 3 years), soonya maasa(sun in pisces, gemini, virgo, sagittarius) and combust days of jupiter and venus, pick a date which satisfies above conditions of lunar day, weekday and moon’s transit in those nakshatras.
On that particular date, pick a lagna(ascendant or rising sign) so that 7th house is vacant.(only moon closer to fullmoon day is exempted in few cases).
Taurus, Gemini, Cancer, Virgo, Libra, Sagittarius and Pisces are best lagnas for marriage.
Jupiter or Venus should be in lagna or aspecting it(the best possible time).
All evil planets like Rahu, Ketu, Saturn, Sun, Mars should be avoided placement or aspect on 7th house.
Mars should not be placed in 8th house of muhurtham and Venus should not be in 6th house.
Also, two siblings from same family should not get married on same date.
Consult your personal astrologer to make sure that you don’t have bad transits or bad dasas running while marriage or starting within short time after marriage, because they might lead to seperations and divorce.