Chandra Maas
From one Amaavasyaa to another Amaavasyaa, the month is called "Chaandra Maas" (Lunar month).

Saavan Maas
30 Days are in Saavan Maas.

Saur Maas
From one Sankraanti to another Sankraanti is "Saur Maas" (Solar month).

Nakshatra Maas
From the 1st Nakshatra (constellation) to the end of 27th Nakshatra, when all Nakshtra are finished, is called "Nakshatra Maas".

Marriage etc are considered by Saur Maas;
Yagya etc are considered by Saavan Maas; and
annual Shraaddh etc are considered by Chaandra Maas.

One must do Pitar Shraaddh in the 5th Paksh (fortnight) which falls after Aashaadh Maas (means in the first half of Aashwin Maas - this is called Pitra Paksh). There is no need to consider Soory's position at that time - that whether Soorya has entered Kanyaa Raashi (Virgo Sign) or not.

In a monthly or annual Vrat, if a Tithi falls on 2 days, then the Tithi falling on the 2nd should be considered auspicious.

In Nakshatra Vrat, one should keep fast on the same Nakshatra in which Soorya sets.
In day fast, one should keep fast on the same day when the Tithi is spread up the whole day;
While in Nakt Vrat, one should consider the Tithi prevailing at night.

When 2nd meets 3rd; 4th meets 5th; 6th meets 7th; 8th meets 9th; 11th meets 12th; and 14th meets Poornimaa and Amaavasyaa meets 1st - these Yog are auspicious. The conjunction of these Tithi gives great fruits.
On the contrary, when 1st meets 2nd, 3rd meets 4th; 5th meets 6th; 7th meets 8th; 9th meets 10th; Yog are terrible. It destroys all Punya done previously.
3rd, 6th, 8th, 11th, and 14th - if these Tithi are with their previous Tithi, one should not keep fast on that day. When they are with the next Tithi, ie 3rd with 4th, 6th with 7th, 8th with 8th, 11th with 12th, and 14th with Poornimaa or Amaavasyaa; then only they should be observed. If there is Ekaadashee on previous day and night, and it is next day also for half an hour, then one should keep fast on second day, not on the first day.
[Padm Puraan, 5/4]

If a woman has taken up a long Vrat, and if her periods appear during that period, that doesn't hinder her Vrat.
A pregnant woman; a woman in Sootak; a menstruated woman should take help of others to do any auspicious work.
A man, if unable to do any Vrat, should take help of his wife or daughter to do it.
When a Vrat has been started, it should be continued even in Sootak and Paatak; only worshipping should be stopped.
If a Vratee becomes unconscious because of Vrat, he can be given milk etc or use any other way to bring him in the senses.

A Vrat is not broken by these 8 things - water, fruits, roots, milk, Havishya (Ghee), fulfilling Braahman's wish, Guru's order, and medicine.

In all kinds of Vrat, it is customary that the Vratee should worship the Deity's idol after taking bath and sleep on the floor in the night. At the end of the Vrat, one should do Jap, Havan, and Daan. Along with this one should feed 24, 12, 5, 3 or 1 Brahman and give Dakshinaa to all.
[Agni Puraan, 19]

The following things do not spoil Vrat - Havishya taken by the permission of Braahman, milk, medicine taken by the permission of Guru, water, roots, and fruits.
[Agni Puraan, 23]