The forty samskaras which are meant to purify the individual self are: garbhadhana, pumsavana, simanta, jatakarma, namakarana, annaprasana, caula, upanayana, the four rites like prajapatya (Vedavratas) performed during gurukulavasa (the years the celibate student spends in the home of his guru), the ritual bath on completion of gurukulavasa, marriage, the five mahayajnas performed everyday by the householder. We have listed nineteen so far. Then there are seven pakayajnas, seven haviryajnas and seven somayajnas to be conducted by the householder. Thus 19+21=40
The seven pakayajnas are: astaka (anvastaka), sthalipaka, parvana, sravani, agrahayani, caitri, asvayuji. The seven haviryajnas: agniyadhana, agnihotra, darsa-purnamasa, agrayana, caturmasya, nirudhapasubandha, sautramani. The seven somayajnas: agnistoma, atyagnistoma, uktya, sodasi, vajapeya, atiratra, aptoryama.
Out of the forty samskaras some are to be performed everyday, some at certain times and some at least once in a lifetime. In the first category there are five mahayajnas (panca-mahayajnas).
Rites done to the chanting of mantras are more beneficial than those done without it - a sacrament involving mantras is a samskara. The social service that a house holder does is included among his daily panca - mahayajnas. The panca - mahayajnas are: brahmayajna, devayajna, pitrayajna, manusyayajna and bhutayajna. The chanting of the Vedas constitutes brahmayajna. Sacrifices and puja are devayajna. Tarpana is pitrayajna. Feeding guests is manusyayajna. And offering bali to various creatures is bhutayajna.
Aupasana and agnihotra are part of the daily religious routine. Though a pakayajna, aupasana is not included in the group of seven pakayajnas mentioned above, while agnihotra is one of the seven haviryajnas. Darsa - purnamasa is a haviryajna to be performed once in fifteen days. The other five haviryajnas and the seven somayajnas are to be performed once a year or, at least, once in a lifetime. As if out of consideration for us, the Smrtis have granted us this concession: that the difficult somayajnas need be undertaken only once in a lifetime.
But for the parvani - sraddha which is to be performed once a month and the sthalipaka every Prathama, the other five pakayajnas are to be performed once a year.
To put it differently: the five mahayajnas (brahmayajna, devayajna, pitrayajna, manusyayajna, bhutayajna) together with agnihotra and aupasana are to be performed everyday; darsa-purnamasa and sthalipaka once a fortnight; parvani-sraddha once a month. The other yajnas are to be conducted once a year or at least once in a lifetime.
On a plot of land growing one crop, harvesting is done once a year, while on another plot growing three crops the same is done once every four months. Some crops have to be watered everyday, some on alternate days. Such jobs are samskaras. But there are differences in the samskaras for different crops. The same is the case with the samskaras meant for human beings.
For a general background, please see here