While one readily understands the concept of masikam specifically related to tithi, I am not able to fully understand the significance of the oona masikam. It is not directly related to tithi but to some vague range of allowable dates in and around the 27th, 45th, six months and a year. Could experts guide us: what is the purpose and significance of this oonam, why do we have such an imprecise method of choosing a day for it when everything else about sraadha karyams is so precise, and is it of higher importance than even the monthly masikams, and if so, why, and what are the prayoga differences between a oona masikam (say 6 monthly), and a masikam which may occur just two days later?