Avengers comes out on the 4th, Solo on the 25th, so it’s more a beginning/end of the month thing. I think Disney think that Avengers Infinity War is going to make the bulk of its money in those first three weeks like the first two did, and that by the 25th their audience will be more word-of-mouth and repeat viewers, whereas the Solo audience in the first couple of weeks will be first time viewers.
Also, they own both franchises – the dollars all go into the same pocket at the end of the day.