You could use a combination of rand and randbetween to get the answer you want
The function first generates a random number between 0,1. If the number is less then 0.75 (75% of the time) it uses the first randbetween function, if its higher it uses the other one.
The randbetween takes your start and end dates and returns a random date in that range.
Note the way the function is setup assumes the date ranges are mutually exclusive and that your high probability dates are at the start (or end) of the timeline. If its in the middle you would need to adjust to take a third randbetween function.