In Pride and Prejudice, what was the date or time of year when Mr. Collins showed Elizabeth Lady Catherine's estate?

The answer to this question can be found in Chapter 26, in
which Charlotte Lucas takes her leave from her friends and family and goes with her
husband-to-be. Lizzie is rather surprised to see that Charlotte desires her to visit her
very soon, and in fact implores her to join her father and sister in their first visit
to her. Note what she says:

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"My father and Maria are to come to me in
March," added Charlotte, "and I hope you will consent to be of the party. Indeed, Eliza,
you will be as welcome to me as either of
them."

Thus we can see that
Lizzie's visit to Charlotte and Mr. Collins happens in March, which is also when Lizzie
is introduced to Lady Catherine de Bourgh and shown her estate for the first time. This
of course is a vital element in the plot of the novel, as it allows Lizzie to spend more
time with Darcy and also to get to know his aunt, the formidable Lady Catherine de
Bourgh.