Probably early morning or evening, when it's not going to be too wet. Probably better not to do it on a windy day also.
I wouldn't do it in the morning if you're expecting that day to be very hot and very dry low humidity, which is often the case in some climates. However, in Germany, the issue would be more cold, so all this wouldn't apply. If it's outside, you may want to wait until May to pollinate (not that you may have much choice because they will flower when they flower). So in Germany it might be better to try it in the late morning.