The Economist's Trump vs Biden polling average was updated today, March 21st, and shows that their average of the polls as of March 17th was Biden 45%, Trump 44%.

Will Trump retake the lead before the end of the month?

If the average for any day from March 18th-March 31st shows Trump ahead of Biden, this market resolves YES. If Biden remains ahead or tied for each of these days, this market resolves NO after the last day of March is added to the average.

Note that due to the delay, we should expect not to see the data for March 31st until a few days after that in April.

You can find more questions like this on the polling dashboard.