In Act 3, what did Cecily do and pretend Algernon did it?

This is essentially correct,

"On the 14th of February last. Worn out by your entire ignorance of my existence, I determined to end the matter one way or the other, and after a long struggle with myself I accepted you under this dear old tree here. The next day I bought this little ring in your name, and this is the little bangle with the true lover's knot I promised you always to wear."