Marriage Mahurat depends upon multiple things and some of the important things are given below.
Basically you have to check the horoscope of the boy and the girl to see on which dates of marriage is coming for both. For example for a Boy there could be marriage dates in April but not for the Girl. So the first step is to check if marriage is coming for both. If marriage is coming for both then you need to check if the transit is auspicious for marriage. Generally marriage is prohibited when Venus or Jupiter is combust or ast and also certain Hindu months when marriage is not done.
After finalizing the date there is few more things that you need to check which includes looking at Auspicious marriage timing for taking marriage vows.
As suggested the above is highly time consuming and If you want auspicious marriage dates then you can always look at the Paid services from the link below.