This Nandana year, the Mahalaya Paksha starts from Sept 30, 2012.
How to observe?
But, of course, Mahaalaya paksha has some lenience and alternatives, unlike Pratyabdhika sraaddha. No dosham will occur if one opts for some permitted alternatives here. No one need to worry on this count. Paarvana Vidhanam, ie with Homam, is not mandatory here; and it is also observed in parts of our Tamilnadu like North Arcot some people perform only tarpanam, not even hiranya, in this Paksha; it is their desacharam.
Hiranya Sraaddha (without homam):
In general, I recommend doing this Mahalayam as hiranya rupam with five (or six as the case may be) Brahmanaas which involves the following:
1. Offering Rice, Dhal and Plantain (Banana)
2. Offering Thulasi, Chandanam etc
3. Offering Sambhavanai
At the end of this Hiranya Rupa Sraaddha, there will be a mandatory tarpanam also.
Of course, to whomever it is possible and convenient, they can do nitya tarpanam during all the days of this pitru paksha in addition to observing one hiranya sraaddha on any convenient day. Here I may point out that though Paksha means only 15 days, this year Mahalaya Paksha has 16 days. Here again for those who wish to observe nitya tarpanam, they should perform it for 17 days in total. Those who wish to to this nitya tarpanam, better consult their vadhyar for exact procedures and sankalpams. Of course they can consult me also in person-not through net or email.
This Hiranya rupa Sraaddha also has tarpanam as part of it in which one can offer tarpanam for Karunika pitrus also in addition to his own pitrus. Karunika pitrus means, one can do tarpanam not necessarily for his own pitrus alone. Only in this paksha, one can do tarpanam for other persons like Younger brother of father, Elder brother of father, their wives, Brothers (both younger and elder), sons, father's sister, sisters, their sons, wife, in-laws, sister's husband, daughter-in-law, Guru, Boss (yajamaanan), friends etc.
Whatever may be the format one chooses, he has to observe the niyamas/deeksha prescribed throughout the Paksha. Then what it is meant by deeksha? The karta should be in control of himself during the paksha. Control implies eating only at home, and not eating outside except at a brother's place or in his maternal uncle's or in-law' place. Shaving, having an oil-bath and conjugation are to be avoided.
Annarupa Sraaddha (Parvana vidhanam) - at least once.
This Annarupa Sraaddha, though involves a lot of preparations, elaborate procedures and monetary aspects, is highly recommended. One can think of performing it at least once in a life time during this Mahalaya Paksha. Your Vadhyar would able to guide you in this.
Saaddha on which day?
During the Mahalaya paksha one can perform the sraaddha on any convenient day, not necessarily on the day of the so called his father's thidhi. Of course if the date of the Pratyabdhika Sraaddha falls in this Paksha, it is quite possible for some, then they have to perform the Mahalaya sraaddha only after the annual sraaddha, not earlier.
Any particular days:
However, some days are special and more powerful for performing sraaddha during this fortnight. Those days are Maha Bharani, Gajachaaya (thrayodasi), Madhyashtami. In addition to these, Vyatheepadham and Dwadasi days are also considered great.
Any particular day NOT recommended?
Yes. On the day of Chathurdasi thidhi, sraaddha is performed only for those who had dur-maranam. Hence all others to avoid doing Sraaddha on this day.
All sons together.
Though nothing prevents sons joining together on this day. However it is considered better if the sons of a deceased father are living apart, each son has to perform individually.
On a personal note, I may suggest that for whomever it is convenient and affordable they can have Brahmana Bhojanam arranged as part of hiranya sraaddha. This can be carried out on any convenient day during this pitru paksha.