Resolves as Yes if an official IDF spokesperson declares the IDF has entered the city of Rafah en masse in Gaza. Because of the ambiguity of "en masse" and the potential for edge cases, other examples of things that would be strong arguments for Yes:

3+ Major news outlets such as CNN, the BBC, Al Jazeera, or the Washington Post describing the current Rafah operation as an invasion, territory taken, occupation, or taking an area such as western Rafah "under Israeli control".

The map here showing reported Israeli clearing operations across 10% or more of Rafah: 

https://storymaps.arcgis.com/stories/2e746151991643e39e64780f0674f7dd

An overwhelming preponderance of other prediction markets with similar language resolving Yes. Not just markets describing strikes in Rafah, but its full-scale invasion and at least substantial clearance.

If Joe Biden considers his red line on a Rafah invasion to have been crossed, that will be enough for a Yes.

Surgical raids of limited scope will not count, only a large scale clearing operation. If resolution is very murky as the month ends, up to a week will be taken after market close to wait for clearer evidence. If Rafah is invaded in May, this market will resolve No.