In different parts of Thailand, Songkran can be celebrated during different/extended periods. Living in Bangkok, since 2007, I haven't noticed it celebrated beyond the 13th~15th. Because Songkran falls over the weekend, this year, I can't see any reason for the partying to extend beyond.
If you are here, between the 13th and 15th, do expect to get wet, and prepare accordingly. Outside that timeframe, you should be safe.
Because this is a time to visit family, many will head "upcountry" to visit. This can affect vendors, who may not open their stalls and shops, because they are out of town. But, by the 18th, things should be back to normal.
Do remember that Chatuchak is a weekend venue.