If last season ended with the Cuban missile crisis still unresolved and this season opens with Betty showing more than a little, I'm thinking Spring 1963, maybe April-May but surely not later than June.

Think you are right... end of April, beginning May.  How about the Maypole dance and the barefoot girl with "flowers in her hair" (later 60's foreshadow)

yep the Maypole nailed it. 

But now that you mention it, what was Don doing playing with the grass while ogling Sally's teacher?  I didn't follow that.

Reference to the Walenda death..in late April..so May 1st is May day..5/1/63 is where we are now

Didn't Don talk Betty through touching the grass
below her chair in another episode? Sort of an
attempt to get her to relax and try to get in touch with her repressed sensual side? So the grass
thing comes up again. If Don is sitting in an uncomfortable chair out on the lawn wearing a suit, all buttoned up and repressed, he's reaching out for the "wild man" he doesn't want to lose...

I will have to go back and look for that scene - I don't recall it.  So you remember which season?

 

It was sand - she's on a beach (because I'm a whale) - first episode of season 3.