I usually quote the beginning of the wet season as being typically around the last week in May. But that can very by up to a week or two, earlier or later. But also, frequently there is a dry period after the official rainy season has started.
The start of the Rainy season is defined as:
"One criterion for the onset of the rainy season is two consecutive days of rainfall accumulation of 30 mm or more, and four of the following seven days are rain days of 1 mm rainfall or more."
Or put simpler, over 31mm ( 1¼") Rain in a Week, and 6 of the 9 days are rain days, > 1mm.
As you can see here, although rains did start on the 29 May 2018 last year, the official start was 05 Jun followed by a dry period in the middle of June.
Our rains are very erratic.
The first year 2010 shows the long term typical monthly rains... Then you can see how erratic the actual rains are
The light blue line shows the Monthly averages, all over the place each year, not following any pattern.
But the grey line is the running Yearly rainfall and it is surprisingly steady, year after year.
In 2013 the rains started suddenly on the 27 May 2013 with a staggering 156mm ( >6" ) in just about half an hour if I remember correctly, some of Belmopan under 6" of water, but the roads were clear within half an hour after that.