Meno is correct
as this year is a leap year, we start on Dec 5th, ie Thursday night.
The reason for this is as follows:
Vesein tal umatar is said (in chu”l) 60 days after tekufas tishrei
for this calculation we use Tekufas Shmuel, that takes the year as ebing 365.25 days (ie 365 days 6 hrs)
There are 4 tekufos (seasons) each is a quarter of a year ie 91 days 7.5 hours. Tekufas
Thus a season begins 6 hours later each year than in the previous year
Tekufas tishrei is always on October 7. however it moves up by 6 hours ever year
2015 Oct 7 9 PM
2016 Oct 7 3 AM
2017 Oct 7 9 AM
2018 Oct 7 3 PM
2019 Oct 7 9PM
2020 Oct 7 3 AM
Now, in 2016 and 2020. instead of the tekufa moving forward by 6 hours, it moved back by 18 hours. This is becasue there was an extra day in Feb 2016 and 2020 (leap years) thus the following October’s tekufa moved “back” a day add the 6 hours that always moves forward so net is moved back 18 hours.
Now to determine when you say vesein tal umatar count 60 days from October 7 (with Oct 7 being #1)
And you get December 5th.
however the halachic day begins the previous night. Thus last year Dec 5th was Wed, which halachicly begins Tuesday night (when our calendar day is still Dec 4th so in a typical year we start Maariv on Dec 4th.
However every 4th year, the tekufa occurs on Oct 7th at 9 PM. This occurs in years before leap years. such as this year 2019. This year the Tekufa Tishrei was Oct 7 9 PM Halachikly this was Tuesday (although your calendar said Monday) 60 days after Tuesday (Oct 8 on the calendar) is Dec 6th. Thus this year 60 days after the Tekufa is Friday Dec 6th, we begin Vesein tal umatar at the start of this 60th day ie Thursday night when our calendar reads Dec 5th.
(The above holds true in the 20th and 21st centuries. in the 22nd century ie after 2100 chaneg all the date move up one the tekufa will occur on OCt 8, and Vesein tal umatar on the nights preceding Dec 6 and 7 (ie on Dec 5th and 6th)
Int eh 1800’s subtract a date, the tekufa was on OCt 6th and vesein tal umatar started the nigths preceding Dec 4th and 5th (ie on Dec 3rd and 4th)
hope you dont mind the arichus, this is a topic that interests me and people often don;t have it quite right . I beleive this is the accurate, if perhaps technicla and wordy explanation.
please let me know if there is any error