On this wiki, we have Sybbie Branson being born in August/September.
I, now, belive this is wrong and that she may have been earlier.
My reasoning: In episode 3.08 Robert mentions "A chap called Ponzi". Charles Ponzi's scheme collapsed on 11th August, thus episode 3.08 must be before that - and the ratification of the Tennessee amendment that Robert mentions occurs (in 3.04) occurs between 3.08 and the Christmas special.
Thus, the events of 3x04, 3x05, 3x06, 3x07 and 3x08 all must be before 11th August.
We know that 3x06 and 3x07 (which mention and contain the Christening) occur closely together -- and 3x06 itself is not long after 3x05 when Sybil dies, because the baby hardly changes appearance and the cars for Sybil's funeral are seen leaving at the beginning of 3x06.
Also, as seen with Cora in season 1, Sybil had to be at least four months pregnant to know about it when she wrote to Cora in December 1919. Cora was 4 months pregnant in 1x07 before her miscarriage when she was told she was pregnant by Doctor Clarkson and no-one had seen it before that.
This places Sybil falling pregnant around September 1919. Nine months after September 1919 is June 1920.
The Cricket Season for 1920 - which Downton would play by - occurred between 24th April and 18th September (as seen here: http://cricketarchive.com/Archive/Events/23/Yorkshire_Cricket_Council_1920.html)
Thus, my reasoning is that Sybbie was born around June, for the funeral of her mother, her christening and the cricket to all occur in July, certainly no later than the first week of August(approx).
By this, we can concur that, with 3x05 - 3x08 occurring in May and June, that at least a week prior was Edith's aborted wedding to Anthony, which took a month to prepare.
This means that Anthony proposed in April; at that time, Mary and Matthew had just returned from their month's honeymoon - making their marriage in March and Martha's arrival shortly before.
Thus, by this I propose this chart:
3X04: Between May and June
This would also fit with the Christmas special in which they go to Duneagle for the Grouse. The Grouse occurs on September 1st. Your thoughts?