Anyone who has taken up 'birding' in the last twenty years or so will take little egret (Egretta garzetta) as an almost everyday sighting and not unusual at all and will not know that that has not always been the case. In these days of doom and gloom with so many species in decline the little egret is certainly one, along with a small number of other 'water' based species, who are bucking that trend, doing very well and spreading their range in Britain. Apparently you can see little egret as far north as Inverness now. I saw my first one in 1986, ten years after starting bird watching and that was quite an event at the time.
In Dorset you can now see little egret all year round and in all sorts of locations. They can be frequently seen in coastal marshes and along the county's rivers, sometimes in good numbers; I have seen nineteen in a single tree at Middlebere. The distribution map shows just how widely spread they now are and how far inland they can be encountered. The coastal sites are mainly those with saltmarsh (Poole and Christchurch harbours and the Fleet) and inland tend to follow the Stour and Frome river valleys but they can also be found around large inland lakes and reservoirs such as Longham Lakes.
The charts showing when you can see little egret in Dorset are, I believe, quite misleading and are a reflection on the vagaries of Twitter reporting rather than fluctuations in populations due to migrational movement or breeding cycles. Little egret can be seen in favoured locations all year round and their numbers, as far as I am aware, are not rapidly increasing. That said, there are three times as many reports in 2018 than there were in 2017 and there is a surge in reports from July 2018 onwards. A look at where the reports come from I think explains this; Holton Lee has received far more attention in recent times and you cannot go to the hide on the marsh at Holton Lee and not see a little egret feeding; by far the most records come from Holton Lee on the western end of Poole Harbour. Neighbouring Lytchett Bay has a good number of records too along with various other sites within Poole Harbour,
If little egret is on your target list to see then probably your best bet is to head to Arne and follow the Coombe Heath trail and you will see them in the Middlebere Channel at low tide or in nearby trees at high tide.