Most Brissim are done in the morning, as it is mentioned that "Zerizim Makdimim L'Mitzvos". Why is this not applicable to a Pidyon Haben ceremony? If it is applicable, then why have I observed that most people make Pidyon Habens towards evening?
It could be that such a custom exists because the baby needs to be 29.5611 days old and if he was born close to the end of one day, then in the summer when the nights are under 12 hours long, he would not be old enough first thing in the morning. (See R Akiva Eiger to Shach 305:12 and Pitchei Teshuva 305:17 who discuss similar concerns.)
There is an opinion that a full lunar cycle must elapse before doing pidyon. One complete cycle takes about 29.5 days. If the baby was born late in the afternoon, and the pidyon haben ceremony is celebrated early in the 31st day, perhaps a complete lunar cycle has not yet elapsed.
However, sefaradim hold that one should do pidyon as soon as possible, on the night before the 31st day, since the day began the previous night. Chacham Yosef has a great teshuvah on this in Yabiah Omer, somewhere in chelek tet.