Contract rollover starts next Monday and ends Friday. May contract is trading 6% higher than April. So when may contract hits 1.68 due to time decay UGAZ will have lost another ~18% and DGAZ will gain that much minus whatever beta decay it experiences.
Page PS-45 of the UGAZ and DGAZ prospectus shows the rollover dates as the 5th thru 9th business days of the month I believe.