Not sure what you mean. If you mean was the weather the reason for choosing the date my opinion is no. In almost all respects the down pour rain that occurred was all negative with respect to the crime for several reasons.
First, you are going to get wet, if not soaked.
Second, inside the building you would have difficulty hearing any vehicle coming to the church, driving by, and if you are far enough away even hearing someone enter. That is because the church has a metal roof and the pounding rain will produce significant noise. While the ceiling tiles will dampen that to some degree the noise will be there. To get a good idea of what the rain was like at about the time SP is said to have entered the building look at the SWF video when the Altima is parked there as the rain and how much rain was coming down seen at that time was very much the same around the time SP was making entry to the church.
Third, with that amount of rain the drainage systems along the roads in the area especially down towards 287 get overwhelmed with water. Ovilla Rd to the south of the church (one of the possible roads for Missy to have taken from her home) is prone to flooding and road closure as it gets down toward 287. The road to the North of the church (South Walnut something, can't recall) also has issues. So that introduces the time issue. What if Missy runs late because she can only get so far the way she normally goes to the church and then has to turn around and go another way?
Fourth, with the amount of rain that was coming down hydroplaning and standing water become issues, not necessarily for Missy or SP but if an accident were to occur on 287 near the church there could be police right there. Or having completed the murder SP gets outside only to find police right there on the road and you have SP in SWAT gear trying to pass by.
Fifth, that kind of rain that was going on also reduces visibility. Could SP tell if it was Missy's truck? If she was alone? If another car arrived or was already there while SP was in another part of the building prior to the murder?
If this were a planned, targeted killing it is my opinion that you wouldn't choose the date based on the weather. If it were some other situation like a burglary gone wrong or vandalism gone wrong, perhaps.
For what it is worth I believe that the murder was likely planned for a specific date. However, I believe the date was changed to take advantage of the fact that BB was out of town and would, in theory, be on a boat in the Gulf of Mexico by the time LE got around to tracking him down. I also believe that the killing was always going to be on an early Monday morning, just not April 18th but the circumstances related to BB changed that.
