Per a USA Today article which references the Freddie Mac Primary Mortgage Survey:

Mortgage buyer Freddie Mac says the average on the 30-year loan was 3.66%. That’s unchanged from last week and the lowest since long-term mortgages began in the 1950s.

The average rate on the 15-year mortgage, a popular refinancing option, slipped to 2.94%. That’s down from 2.95% last week and matches the record-low 2.94% reached three weeks ago.

The rate on the 30-year loan has been below 4% since December.