My current theory is this:
1) There was that document posted here earlier from the early days of the harmonization program, which said that results would be released to alternates the day after the first offer expiry deadline. That was talking about May 15/16, which seems to have changed.
2) At least going by postings on this forum and Twitter, we know that some schools (U of T, UWO, some others) have already pulled the trigger and contacted alternates (presumably right as first rounders declined), whereas at least some schools seem to have stayed "silent".
3) One possible scenario, therefore, is that some schools have gone ahead and contacted immediately, whereas others have been following the original "day after first round expiry" plan. Given that schools weren't allowed to announce first offers before Apr. 1, maybe some decided to play it safe and not offer second offers until after this date.
4) If so, and depending on how they count 3 weeks from Apr. 1, we should see a number of offers from previously "silent" schools coming out tomorrow or Wednesday. Or really any time after that.
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There are holes in this theory, because it could be that all schools have been contacting alternates all along and those lucky souls just haven't put their success up on the Internet.
At any rate, a few spots across the board could be opening up soon as the first round expiry date passes.
Hang in there, everyone.