Searching on United's website, I found seats in Basic Economy departing DTW on Friday, October 26, and returning from SAN on Monday, October 29, 2018. Flights at this price are available daily, with no minimum stay required. There are a few scattered dates in May and June, but the bulk of availability is from late August through March of next year.
Need a room in San Diego? Here's what's available for the above dates on Hotelwatchdog.