Chapter 2 of Garuda Purana throws light on the monthly ceremonies to be performed for the departed soul.
The fifteen monthly Pindadaan and the unaabdika Pindadaan would have been performed by you for your mother. Her soul has completed her journey to Yamapuri. At this juncture, if the final Pindadaan is not done, the day before the annual ceremonie, then her soul will still be hungry.
It is the duty of the son / grandson to perform the ceremonies for the departed soul. Verses 79 to 83 are reproduced below:
सम्पूर्णे तु ततो वर्षे शीताढयं नगरं व्रजेत् | हिमाच्छतगुणं तत्र महाशीतं तपत्यपि ||
शीतार्तः क्षुधितः सोsपि वीक्षते हि दिशो दश | तिष्ठते बान्धवः कोsपि यो मे दुःखं व्यपोहति ||
किङ्करास्ते वदन्त्यत्र क्व ते पुण्यं हि तादृशम् | भुक्त्वा च वार्षिकं पिण्डं धैर्यमालम्बते पुनः ||
ततः संवत्सरस्यान्ते प्रत्यासन्ने यमालये | बहुभीतिपुरे गत्वा हस्तमात्रं समुत्सृजेत् ||
अङ्गुष्ठमात्रो वायुश्च कर्मभोगय खेचर | यातनादेहमासाद्य सह याम्यैः प्रयाति च ||
Thereafter the year being completed the soul enters the place named Sheetaadhya, where it is a hundred times colder than the coldest place on earth. Suffering from the extreme cold and weak with hunger, the soul looks around hopefully in all ten directions, that may be there is some relative to help me out of this suffering. The messengers of Yama ask the soul, "Where have you acquired such piety (punya)?" Then seeing the annual offering of the pinda, the soul feasts on it and gets further strength. The soul has now reached Yamapuri, it enters the place called Bahubheetepura, where its recreated body of the first ten days offerings is discarded and it takes on its airy body to bear the fruits of its karma enters Yamapuri with the messengers of Yama.
Garuda Purana is the dialouge between Mahavishnu and His mount Garuda. My understanding is that on no condition, except the death of a very near one in the immediate family, the first annual ceremony should be performed without fail. I feel that Yama would not be waiting for the performance of this ritual for the departed soul due to deaths of someone in the extended family.