As a month, May is mostly about delegate selection rather than delegate allocation. Even on the Democratic side (where some caucus allocations will be finalized), there will be over twenty delegate selection events in various states but fewer than ten delegate allocation events.
On the Republican side, there is just one delegate allocation event — Indiana. After a good showing this past Tuesday (Trump even apparently got 31 supporters elected as unpledged district delegates in Pennsylvania), Trump looks to have a shot at getting enough delegates to win on the first ballot. He still needs to win fifty percent of the remaining delegates though (approximately 250). Indiana is another winner-take-most state — three delegates to the winner in each of the nine congressional districts and thirty to the state-wide winner. Indiana is the last best chance for Cruz to prevent Trump from getting the nomination. After trying to arrange a deal with Kasich and (shades of Ronald Reagan) announcing his VP candidate, Cruz has few angles left to play. Trump is up by 6% which would likely give him 45+ delegates. If Cruz can make a comeback (with the help of Kasich supporters), Trump is probably looking at 15 or fewer delegates. With only around 450 delegates left after Indiana, a thirty delegate swing is a big deal.
On the Democratic side, the window is slowly closing on Bernie Sanders. Depending on which count you use (i.e. do you count tentative delegates from Iowa, Nevada, Washington, etc.) Hillary Clinton is between 360 and 450 votes of a majority of pledged delegates. If you include unpledged delegates, Hillary Clinton is within 240 of a majority at the convention.
In Indiana, the nine districts have between five and eight delegates each (fifty six total). The third, fourth, and sixth districts, have five delegates each (almost guaranteed to be a 3-2 split). The second, eighth, and ninth districts have six delegates each (probably a 3-3 split unless one candidate get to 58.4%). The fifth district has seven delegates (probably a 4-3 split). The first and seventh district have eight delegates each (a 4-4 split unless a candidate gets to 56.3%). There are nine party leader delegates (5-4 unless one candidate gets to 61.2%) and eighteen at-large delegates (52.8% to get a 10-8 split) for a total of eighty-one delegates at stake on Tuesday. If Sanders can pull out a win, he is probably looking at 44-48 delegates leaving Clinton to pick up 33+ delegates. (Clinton is currently leading in polling.) While a win is better than a loss, Sanders really needs to get in the mid-60s or better in Indiana to make any significant gains. While the rest of May is probably better for Sanders than this week, he needs a surprisingly good week at some point.
Saturday is the second of three territorial conventions — this time in Guam. Clinton won the first — American Samoa with 68% of the vote. As Guam has seven delegates, a similar result would give Clintin a 5-2 split (it takes 64.3% to bump up from 4-3 to 5-2.)
Also on Saturday is the Maine state convention. Back in early March, Sanders did well in Maine at the township/municipal mass meetings. The big chance for a change on Saturday is at the congressional district level. In the first district, Clinton got the last delegate by 19 out of 1921 delegates. In the second district, Sanders got the last delegate by 13 out of 1499 delegates. Theoretically, the state-wide delegates could also change but Clinton got the last at-large delegate by 187 out of 3,420 delegates. In short, Clinton will get 8-10 delegates out of Maine and Sanders will get 17-15 delegates out of Maine when things are finalized.
In a look ahead, the week of May 9 has West Virginia (primary) and the Nevada state convention (only impact the allocation of the state-wide delegates) on the Democratic side and Republican primaries in West Virginia (all delegates other than automatic delegates directly elected) and Nebraska. The week of May 16 has the Oregon and Kentucky primaries on the Democratic side (along with the Nebraska county conventions) and Oregon on the Republican side. The week of May 23 will have the Wyoming state convention (only impacting the allocation of the state-level delegates) on the Democratic side and the Washington Primary on the Republican side.
In short, May is mostly one state at a time with Indiana being the most significant. On the Democratic side, the issue is not just how many of these contests that Sanders can win but rather by how much he can win them. Any contest in which Sanders gets less than 60% of the delegates brings Clinton closer to the nomination. On the Republican side, the issue is can Trump get 50% of the delegates (and how many over 50%. Each state in which Trump beats 50% is one less district that he needs to win in California on June 7. Each state in which Trump fails short is one more district that he needs to win in California.