I had heard that many people perform a Pidyon Haben on the 31st day after the child's birth.
In viewing the verse that refers to the timing of the Pidyon I see Bemidbar 18:16 which says that one should redeem the first born when he is one month old (בן חודש). This leads to several definitions of what a חודש means, here. Which of these methods should we use to count?
- According to the fixed calendar that we have now, a month may be either 29 days long or 30 days long. Should we make the pidyon on the same Hebrew date of the following month?
- Do we follow the molad definition of a חודש, meaning 29 days, 12 hours and 793 chalakim?
- Do we use שעות זמניות ("seasonal hours") to count each "day"? If so, what if the baby travelled from Israel on the chodesh day and he was eligible for the Pidyon but when he arrives, it is still not yet the chodesh day? OR ...
- Do we use a fixed 24-hour day with each hour being a fixed 60 minutes?
- Some other method
In surmising - is the reason that people wait 31 days a way of "compromising" the doubt caused by the above vagueness?